Indexes: add WHERE and USING

[sashabelozerov]

This pull request is inspired by other PR #422
It adds WHERE and USING values to the index info.
This is really useful feature, and since original PR contributor does not appear since Dec 2016, I decided to push it forward. Hope this is fine.

USING is added only if index is not btree.

Difference from #422:

1. Markdown support
2. Refactored code
3. Added missing specs

Example:

# Indexes
#
#  index_rails_02e851e3b7  (id)
#  index_rails_02e851e3b8  (firstname,surname) WHERE city is not null USING hash

[ctran]

Thanks!