Running ctags -R under home directory is so annoying. It's literally
endless. So I'm adding this whitelist feature.

Usage:

Add global variable g:easytags_whitelist. g:easytags_whitelist is an
array containing regular expressions.

For example, following statement makes easytags run only under path
containing the string "workspace", such as ~/workspace/vim-easytags
    let g:easytags_whitelist = ['workspace']