Add --prefix_header_includes option (fixes issue #112).

--prefix_header_includes controls presence of includes and forward declarations
involving files included via command-line option -include. Issue is reported by
Max Dyckhoff.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@497 812d1f90-f944-d88a-411a-122bdc93e1ea