Credit syntax definition and theme authors with new --acknowledgements option (#1971)

The text that is printed is generated when building assets, by analyzing LICENSE
and NOTICE files that comes with syntaxes and themes.

We take this opportunity to also add a NOTICE file as defined by Apache License 2.0.

src/bin/bat/clap_app.rs

@@ -508,6 +508,12 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                 .hidden_short_help(true)
                 .help("Show diagnostic information for bug reports.")
         )
+        .arg(
+            Arg::with_name("acknowledgements")
+                .long("acknowledgements")
+                .hidden_short_help(true)
+                .help("Show acknowledgements."),
+        )
         .arg(
             Arg::with_name("ignored-suffix")
                 .number_of_values(1)
@@ -578,6 +584,12 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                             "Create completely new syntax and theme sets \
                              (instead of appending to the default sets).",
                         ),
+                )
+                .arg(
+                    Arg::with_name("acknowledgements")
+                        .long("acknowledgements")
+                        .requires("build")
+                        .help("Build acknowledgements.bin."),
                 ),
         )
     }

src/bin/bat/main.rs

@@ -47,9 +47,13 @@ fn build_assets(matches: &clap::ArgMatches) -> Result<()> {
         .map(Path::new)
         .unwrap_or_else(|| PROJECT_DIRS.cache_dir());
 
-    let blank = matches.is_present("blank");
-
-    bat::assets::build(source_dir, !blank, target_dir, clap::crate_version!())
+    bat::assets::build(
+        source_dir,
+        !matches.is_present("blank"),
+        matches.is_present("acknowledgements"),
+        target_dir,
+        clap::crate_version!(),
+    )
 }
 
 fn run_cache_subcommand(matches: &clap::ArgMatches) -> Result<()> {
@@ -324,6 +328,9 @@ fn run() -> Result<bool> {
             } else if app.matches.is_present("cache-dir") {
                 writeln!(io::stdout(), "{}", cache_dir())?;
                 Ok(true)
+            } else if app.matches.is_present("acknowledgements") {
+                writeln!(io::stdout(), "{}", bat::assets::get_acknowledgements())?;
+                Ok(true)
             } else {
                 run_controller(inputs, &config)
             }