Option for mixing dot files when sorting

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