Be more flexible with .cxx matching

[nachogoro]

I have worked on several codebases which use .h as header file for .cxx files. The plugin currently does not support that. Do you reckon it's a common enough situation that it might be worth updating the plugin so that:

au BufEnter *.cxx  call s:SetVariables('hxx,h',        'reg:/src/include/,reg:|src|include/**|,ifrel:|/src/|../include|')
...
au BufEnter *.h    call s:SetVariables('c,cpp,cxx,m',  'reg:/include/src/,reg:/include.*/src/,ifrel:|/include/|../src|')

?