Add <Leader>t to toggle tab expansion.
Add function to cycle between 2/4/8 spaces for tabs.
Merge branch 'ps.add_vim_tab_toggle'

This branch adds two new leader mappings:

<Leader>t
    This mapping will toggle tab expansion. By default it is enabled and
    all tabs are expanded to tabs. In the other way, tabs will not be
    expanded.

<Leader>T
    This mapping will cycle between displaying tabs as 2, 4, and 8
    spaces.