Add missing module

[fabaff]

Looks like that Sphinx is not enough.

(esphomedocs) [fab@localhost esphomedocs]$ make html
sphinx-build -M html "." "_build"  
Running Sphinx v1.7.5

Extension error:
Could not import extension breathe (exception: No module named 'breathe')
make: *** [Makefile:13: html] Error 2

[OttoWinter]

Ah yes, completely forgot that. Thanks 😺

(I think even with breathe installed, it will generate tons of warnings because doxygen isn't set up - I'll put a note about this in the docs)

[fabaff]

Yes, there are warnings and the process fails.

$ make html
sphinx-build -M html "." "_build"  
Running Sphinx v1.7.5
making output directory...
[...]
Exception occurred:
  File "/home/fab/Documents/repos/esphomedocs/lib64/python3.6/site-packages/breathe/directives.py", line 696, in get_mtime
    raise MTimerError('Cannot find file: %s' % os.path.realpath(filename))
breathe.directives.MTimerError: Cannot find file: /home/fab/Documents/repos/esphomedocs/_doxyxml/index.xml