Introduce LaTeX-to-Unicode "smart" tab mode

[carlobaldassi]

This introduces a mode (activated with the option let g:latex_to_unicode_smart_tab = 1) in which the <Tab> and <S-Tab> keys can be used to navigate the menu suggestions produced by the L2U plugin, while <CR> is used to confirm the selection. These keys fall back to whatever behavior they had before if no menu is visible, or if it was not created by L2U.

This PR is to get some general feedback (should it behave differently in other circumstances? Any issues with other plugins? etc.)