Disable autosuggest if buffer is too large

[lbolla]

This is a workaround when copying large amount of texts in a terminal, i.e. with mouse-button-2.
The idea is to disable autosuggestion when buffer is too large. This avoids that the autosuggest function is called too many times (it's called once per character), slowing it down the paste.
I hardcoded the max buffer length to 20 chars. Possibly, it could be a configurable value instead.

[ericfreese]

I'd be open to merging this in if you make it a configurable field with a default of no limit. Can you make that change, add a test to cover it, and submit a separate PR, this time to the develop branch instead of master?

Thanks!

[lbolla]

Submitted new PR #178.