Add index analytics.

Summary:
 - implement an interface for analyzing all expressions in an index
 - provide a drop-in build system for analytics implementations in
 mws/analytics/
 - closes #18

Test Plan:
bin/mws-index -I data/zbl/ -o data/zbl-index
bin/analytics/analyze_example -I data/zbl-index/

Reviewers: radu.hambasan

Differential Revision: http://phab.code4fun.de/D83