WIP: toggle completion caching whether number of candidates reaches limit

[kermorgant]

if the number of suggested candidates reaches the limit, the desired candidate might be missing.

If that occurs, caching should not be used when typing more characters of the completed string, as phpactor will be invoked with a longer prefix, and have more chances to return the desired candidate.

fixes #50

EDIT : this still needs to be tested. I'll try to use test this in a near future by lowering phpactor's completion.completor.class.limit value in order to trigger that code more often