Add method capture_pane toPane

[ritiek]

This new method Pane.capture_pane will be very handy for capturing text from a tmux pane.

>>> pane.capture_pane()
['lots and...',
 '..lots of..',
 '.cute text.']

So kawaii. :D

[codecov-io]

Codecov Report

Merging #120 into master will decrease coverage by 0.06%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
- Coverage   79.81%   79.74%   -0.07%     
==========================================
  Files           8        8              
  Lines         857      859       +2     
==========================================
+ Hits          684      685       +1     
- Misses        173      174       +1

Impacted Files	Coverage Δ
libtmux/pane.py	85.48% <50%> (-1.19%)	⬇️

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[tony]

Nice job.