[VIM] Fix previously broken `Alt-FX -> Alt-X`

Original mappings were `FX` actually without Alt keys, so previous patch made
behavior pretty shitty. Instead of adding option to have Alt+X keybindings for
NerdTree and TagBar in both normal and insert mode, I decided to avoid option
to provide a sugar to insert mode (exit insert mode is not that difficult) and
user <Leader> instead of modifiers.