add an option to load vim-clang only if .clang file is found

User may want continue to use ctags based plugins when he does not have
set a .clang file in its project. When the new option
g:clang_load_if_clang_dotfile is set, it disables the loading of the
vim-clang plugin.

This option searches for g:clang_dotfile and g:clang_dotfile_overwrite
files.