Automatically setup yasnippet if available
add customization options to allow disabling yasnippet integration
Make sure yasnippet doesn't get loaded if the integation is disabled
Update installation instructions
Removed debugging and fixed typo in readme
Clean up recent changes

* Always require `ansi-color`.
* Remove `rspec-if-feature`, use `require` directly.
* Install snippets once, in `eval-after-load`.
* Turn on yas-minor-mode inside rspec-mode function body.

Refs #38