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

@@ -397,9 +397,38 @@ inside of ripgrep.
 Note that this must be passed to every invocation of rg.
 Type settings are NOT persisted.
 .RS
+.IP
+.nf
+\f[C]
+\ \ Example:\ `rg\ \-\-type\-add\ \[aq]foo:*.foo\[aq]\ \-tfoo\ PATTERN`
+\f[]
+.fi
 .PP
-Example:
-\f[C]rg\ \-\-type\-add\ \[aq]foo:*.foo\[aq]\ \-tfoo\ PATTERN\f[]
+\-\-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:
+.IP
+.nf
+\f[C]
+\ \ `\-\-type\-add\ \[aq]src:include:cpp,py,md\[aq]`
+\f[]
+.fi
+.PP
+Additional glob rules can still be added to the src type by using the
+\-\-type\-add flag again:
+.IP
+.nf
+\f[C]
+\ \ `\-\-type\-add\ \[aq]src:include:cpp,py,md\[aq]\ \-\-type\-add\ \[aq]src:*.foo\[aq]`
+\f[]
+.fi
+.PP
+Note that type names must consist only of Unicode letters or numbers.
+Punctuation characters are not allowed.
 .RE
 .TP
 .B \-\-type\-clear \f[I]TYPE\f[] ...

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