Add optional parameter literal to Pane.send_keys

[ritiek]

Closes #88.

This PR adds an optional parameter literal=False to Pane.send_keys which does not send keys to a tmux pane literally by default. When literal is set to True, it prefixes the send-keys command with -l parameter which allows sending keys literally.

[codecov-io]

Codecov Report

Merging #121 into master will decrease coverage by 0.06%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   79.49%   79.42%   -0.07%     
==========================================
  Files           8        8              
  Lines         863      865       +2     
==========================================
+ Hits          686      687       +1     
- Misses        177      178       +1

Impacted Files	Coverage Δ
libtmux/pane.py	80.88% <75%> (-0.94%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7a7027c...122d68e. Read the comment docs.

[tony]

@ritiek Can you rebase this (so the tests again?)

[ritiek]

Rebased. :)

[Seanmatthews]

Seems the only error for this was a timeout? Can we rerun the test?

[ritiek]

Ok, they passed.

[tony]

@ritiek Can we add a test for this?

[tony]

@ritiek merged! Thank you!