Implementation of 'bat --diff'

This adds a new `--diff` option that can be used to only show lines
close to Git changes (added/removed/modified lines). The amount of
additional context can be controlled with `--diff-context=N`.

closes #23

src/bin/bat/app.rs

@@ -15,7 +15,7 @@ use console::Term;
 
 use bat::{
     assets::HighlightingAssets,
-    config::Config,
+    config::{Config, VisibleLines},
     error::*,
     input::Input,
     line_range::{HighlightedLineRanges, LineRange, LineRanges},
@@ -196,13 +196,23 @@ impl App {
                     }
                 })
                 .unwrap_or_else(|| String::from(HighlightingAssets::default_theme())),
-            line_ranges: self
-                .matches
-                .values_of("line-range")
-                .map(|vs| vs.map(LineRange::from).collect())
-                .transpose()?
-                .map(LineRanges::from)
-                .unwrap_or_default(),
+            visible_lines: if self.matches.is_present("diff") {
+                VisibleLines::DiffContext(
+                    self.matches
+                        .value_of("diff-context")
+                        .and_then(|t| t.parse().ok())
+                        .unwrap_or(2),
+                )
+            } else {
+                VisibleLines::Ranges(
+                    self.matches
+                        .values_of("line-range")
+                        .map(|vs| vs.map(LineRange::from).collect())
+                        .transpose()?
+                        .map(LineRanges::from)
+                        .unwrap_or_default(),
+                )
+            },
             style_components,
             syntax_mapping,
             pager: self.matches.value_of("pager"),

src/bin/bat/clap_app.rs

@@ -105,6 +105,34 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                             data to bat from STDIN when bat does not otherwise know \
                             the filename."),
         )
+        .arg(
+            Arg::with_name("diff")
+                .long("diff")
+                .help("Only show lines that have been added/removed/modified.")
+                .long_help(
+                    "Only show lines that have been added/removed/modified with respect \
+                     to the Git index. Use --diff-context=N to control how much context you want to see.",
+                ),
+        )
+        .arg(
+            Arg::with_name("diff-context")
+                .long("diff-context")
+                .overrides_with("diff-context")
+                .takes_value(true)
+                .value_name("N")
+                .validator(
+                    |n| {
+                        n.parse::<usize>()
+                            .map_err(|_| "must be a number")
+                            .map(|_| ()) // Convert to Result<(), &str>
+                            .map_err(|e| e.to_string())
+                    }, // Convert to Result<(), String>
+                )
+                .hidden_short_help(true)
+                .long_help(
+                    "Include N lines of context around added/removed/modified lines when using '--diff'.",
+                ),
+        )
         .arg(
             Arg::with_name("tabs")
                 .long("tabs")
@@ -339,6 +367,7 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                 .takes_value(true)
                 .number_of_values(1)
                 .value_name("N:M")
+                .conflicts_with("diff")
                 .help("Only print the lines from N to M.")
                 .long_help(
                     "Only print the specified range of lines for each file. \