Add gitbranch option

[rasbt]

Adds an option to display the current Git branch via the -b and --gitbranch flags as suggested in #36 .

E.g.,

%load_ext watermark
%watermark -d -b

2018-01-17 
Git branch: test-branch

[ianozsvald]

Using %watermark -d -m -v -p numpy,matplotlib,pandas -g -b and your githash branch, I can run watermark with no git repo:

The watermark extension is already loaded. To reload it, use:
  %reload_ext watermark
2018-01-18 

CPython 3.6.4
IPython 6.2.1

numpy 1.14.0
matplotlib 2.1.2
pandas 0.22.0

compiler   : GCC 4.8.2 20140120 (Red Hat 4.8.2-15)
system     : Linux
release    : 4.9.75-040975-generic
machine    : x86_64
processor  : x86_64
CPU cores  : 8
interpreter: 64bit
Git hash   : 
Git branch   :

If I add a git init:

Git hash   : 61e46b3582776899406cfd939c3de195320dedac
Git branch   : master

If I then make a branch git checkout -b ian then watermark then I see:

Git hash   : 61e46b3582776899406cfd939c3de195320dedac
Git branch   : ian

I'm using

git version 2.7.4

This all looks pretty sensible.

[rasbt]

Thanks for checking! I just see there are two annoying white space characters before the colon. Will fix that and then it's prob. good to go :)