Support LaTeX-to-Unicode tab expansion

This adds support using tab to complete LaTeX symbols in ViM like in the Julia REPL.
However, in order to do so, it may need to interact with other plug-ins installed in the system.

The default (vanilla) behaviour is that upon pressing TAB a latex entry is searched for before the cursor, but if it's not found a literal TAB is inserted. I did not add any mapping to explicitly force a literal tab insertion, which however should always be available with `^V`+TAB.
The substitution is assigned to the omnicompletion setting, so it's also available with `^X^O`.

I'm a user of [supertab](https://github.com/ervandew/supertab) and it seems everything works great with that, but perhaps other users may have custom settings which are conflicting, or get overridden etc., in which case please do let me know.

Another popular plug-in I know about is [YouCompleteMe](https://github.com/Valloric/YouCompleteMe), which at the moment is incompatible with this setting, since it overrides the TAB key binding. At the moment, it seems that the options are 1) change the YCM settings to free up the TAB key for the Julia plug-in, or use `^X^O` to force the Julia omnicompletion to kick in and do the LaTeX-Unicode translation.

Any user of these or other completion plug-ins, or which have remapped the tab key somehow, are encouraged to give it a try, provide suggestions etc. If there are no major problems or objections I shall merge this as is, and then it can be refined later.