Option for mixing dot files when sorting

NERDTree can now optionally ignore the dot at the beginning of hidden
filenames for sorting to show them next to normal files if
`g:NERDTreeSortHiddenFirst` is set to 0.  (By default it's set to 1 to
preserve the current behavior.)  This is just like what GNU ls does when
`LC_COLLATE` environment variable is set to `en_US`.