Add use_vcr_placeholder_for test helper method

Makes it easier to setup cassette placeholders by calling
use_vcr_placeholder_for(text, replacement) instead of doing all the VCR
configure blocks.
Filter private tokens from Feeds specs on record.
Replace vcr configure block with helper method call
Merge pull request #430 from joeyw/more-vcr-placeholding

Add use_vcr_placeholder_for test helper, filter feed tokens on record
Clarify #repositories docs. Fix #449.

[ci skip]