Don't drop words with low certainty

[amitdo]

Fix #681.

[amitdo]

@theraysmith, @jbreiden,
Please review and approve this PR. I want this in to be in Ubuntu 18.04.

[zc813]

Hi amitdo, I encountered the same problem. Is there anything I can do to help with this pull request?

[amitdo]

I think it's a good idea to test it on several languages and variety of pages.

Apart from this, someone with the right permissions will need to merge it...

[zc813]

@amitdo Some of the results are better on Tibetan. Previously missing words are recognized after this commit.
However, there are still several entire lines of text missing. Different lines are skipped if a different psm (3, 6, or 11) is used. It's like #538 , but in my case, all texts are in the same size.

Any idea? Thanks a lot!