Add whitespace settings to modeline

People have their preferred indentation method, but this file is mostly
using 4 spaces for indents. Add a modeline to enforce this.

Fix the few lines indented with tabs.
Expose Ctrl mappings as <Plug> mappings

Users who use Gitv_DoNotMapCtrlKey may want to provide their own
mappings for these commands. Define Plug mappings so users can map them
in ~/.vim/after/ftplugin/gitv.vim. Map the original commands to the
Plugs if enabled.
Spelling: sanatizedArgs -> sanitizedArgs
Remove outermost quotes around Gitv arguments

Instead of only checking for a quoted empty string, remove the outermost
quotes -- they should never be necessary because git doesn't require the
whole string to be quoted.

Fixes #80 which is caused by specific behavior from setting 'shellslash'.

Alternative fix is to remove shellescape.
Merge pull request #83 from idbrii/fix-80-shellslash

Fix #80: make gitv work with shellslash and Windows git