Change the default for localvimrc_persistence_file

Use a hidden file by default.

This might not work as expected on Windows?! Should it use `_` instead
of `.` as with the default for `~/_vimrc`?

Ref: https://github.com/embear/vim-localvimrc/issues/19
Merge pull request #20 from blueyed/localvimrc_persistence_file_default_hidden

Change the default for localvimrc_persistence_file