Prv leader fix

[rfabbri]

A proposed fix for the case where the default leaders are being used,
and are not defined in vimrc. I need a review from @ttm regarding the proper handling of prvset.
I noticed that @ttm 's vimrc has all the leaders in prvset explicitly set. But if the user don't set them, they should be sensible defaults. The defaults are set in PRVInit/PRVDefineSettings but I don't know if they are sufficient to get sensible defaults; they seem like a stub:

  if !exists("g:prvset") " for user settings
    let g:prvset = {'leaders' : {}}
  en

At least my basic idea of setting the s: version of the leaders to the defaults is a proposed solution, even though it needs to be polished.

Moreover, I would need @ttm to check my changes in lines 665,666,667 if I am handling correctly the case when prvset.leaders.prv is undefined.

[rfabbri]

Please read the ideas for a better fix here: https://rfabbri.github.io/aa/