Syntax: add cmd-help (#2148)

* Add cmd-help syntax

To highlight command --help messages.

* README.md: mention help message highlighting

* README.md: edit help message highlighting

tests/syntax-tests/highlighted/cmd-help/test.cmd-help

@@ -0,0 +1,54 @@
+bat 0.20.0 (e735562-modified)
+A cat(1) clone with syntax highlighting and Git integration.
+
+USAGE:
+    bat [OPTIONS] [FILE]...
+    bat <SUBCOMMAND>
+
+OPTIONS:
+    -A, --show-all
+            Show non-printable characters (space, tab, newline, ..).
+
+    -p, --plain                          Show plain style (alias for '--style=plain').
+    -l, --language <language>            Set the language for syntax highlighting.
+    -H, --highlight-line <N:M>...        Highlight lines N through M.
+        --file-name <name>...            Specify the name to display for a file.
+    -d, --diff
+            Only show lines that have been added/removed/modified.
+
+        --tabs <T>                       Set the tab width to T spaces.
+        --wrap <mode>
+            Specify the text-wrapping mode (*auto*, never, character).
+
+    -n, --number
+            Show line numbers (alias for '--style=numbers').
+
+        --color <when>                   When to use colors (*auto*, never, always).
+        --italic-text <when>             Use italics in output (always, *never*)
+        --decorations <when>
+            When to show the decorations (*auto*, never, always).
+
+        --paging <when>
+            Specify when to use the pager, or use `-P` to disable (*auto*, never,
+            always).
+    -m, --map-syntax <glob:syntax>...
+            Use the specified syntax for files matching the glob pattern
+            ('*.cpp:C++').
+        --theme <theme>                  Set the color theme for syntax highlighting.
+        --list-themes                    Display all supported highlighting themes.
+        --style <components>
+            Comma-separated list of style elements to display (*auto*, full, plain,
+            changes, header, grid, rule, numbers, snip).
+    -r, --line-range <N:M>...            Only print the lines from N to M.
+    -L, --list-languages                 Display all supported languages.
+    -h, --help                           Print this help message.
+    -V, --version                        Show version information.
+
+ARGS:
+    <FILE>...    File(s) to print / concatenate. Use '-' for standard input.
+
+SUBCOMMANDS:
+    cache    Modify the syntax-definition and theme cache
+
+Note: `bat -h` prints a short and concise overview while `bat --help` gives all
+details.