Add first iteration of doxygen_utility.

Add:
	vim/bundle/doxygen_utility
		-Add dir for `doxygen_utility` plugin.

	vim/bundle/doxygen_utility/plugin/doxygen_utility.vim
		-Add first iteration of the plugin.
		-Add main `GenerateDoxygenComment()`, which calls all other local-scoped
		helper functions to identify whether the user's cursor is on a line
		containing Doxygen-documentable code, and, if it is, inserts an
		appropriate comment template.
		-Add functional Doxygen-documentable object detection for functions,
		macros, file headers, and structs. Not groundbreakingly robust, but
		sufficiently advanced for general use.
Add macro/function comment generation.

Add:
	vim/bundle/doxygen_utility/plugin/doxygen_utility.vim
		-Add `GenerateMacroComment()` and `GenerateFunctionComment()`, which
		extract the macro and function definition/declaration respectively
		from under the cursor and insert a `Doxygen` comment string with
		appropriate specifiers (`@param`, `@return`) depending on its content.
		-Prevent `IsOnFunction()` regex from ignoring `static` functions, which
		were erroneously excluded.
Add struct/file-header comment insertion.

Add:
	vim/bundle/doxygen_utility/plugin/doxygen_utility.vim
		-Define `InsertStructComment()` and `InsertFileHeaderComment()`.
		-Add missing documentation comments to several functions.
Complete doxygen_utility.vim.

Add:
	vim/(bundle/doxygen_utility/plugin > scripts)/doxygen_utility.vim
		-Move the plugin from `bundle/` to `scripts/`, because the latter's
		associated with others' scripts and plugins.

	vim/scripts/doxygen_utility.vim
		-Add `Doxygen` directive abbreviations.

	vim/ftplugin/c.vim
		-Move `Doxygen` syntax matches from the c ftplugin file to
		`doxygen_utility.vim`.
Commit current state of doxygen_utility branch.

Add:
	vim/ultisnips/*.snippets
		-Minor snippet additions.
Merge branch 'feature/doxygen_utility'

Merge the branch where the `doxygen_utility` script was developed.