Provide a mechanism to compose type definitions

This extends the syntax of the --type-add flag to allow including the globs of
other already defined types.

Fixes #83.

doc/rg.1.md

@@ -267,7 +267,25 @@ Project home page: https://github.com/BurntSushi/ripgrep
   this must be passed to every invocation of rg. Type settings are NOT
   persisted.
 
-      Example: `rg --type-add 'foo:*.foo' -tfoo PATTERN`
+          Example: `rg --type-add 'foo:*.foo' -tfoo PATTERN`
+
+      --type-add can also be used to include rules from other types
+      with the special include directive. The include directive
+      permits specifying one or more other type names (separated by a
+      comma) that have been defined and its rules will automatically
+      be imported into the type specified. For example, to create a
+      type called src that matches C++, Python and Markdown files, one
+      can use:
+
+          `--type-add 'src:include:cpp,py,md'`
+
+      Additional glob rules can still be added to the src type by
+      using the --type-add flag again:
+
+          `--type-add 'src:include:cpp,py,md' --type-add 'src:*.foo'`
+
+      Note that type names must consist only of Unicode letters or
+      numbers. Punctuation characters are not allowed.
 
 --type-clear *TYPE* ...
 : Clear the file type globs previously defined for TYPE. This only clears