Revert |f behavior for 0.7.1.

This reverts the behavior of the `|f` helper to what it was before b40ccfcba362092a79001aa5c0da77fd904ed692, per the conversation in #54. Also adds tests to prevent regressing the behavior of either `|f` or `|fe`.

Fixes #54.
Proposed docstring modification: ```Uses ``str.format`` for string interpolation with (escaped if unsafe) arguments, then return a safe string.```