Allow reading list of connections

[bairdj]

Fixes #509

This amends standardise_path to work with a list of connections, as referred to in the vignette.

The test for this functionality "vroom works with many connections" was incorrectly passing a character vector of paths to vroom instead of a list of connections, so this wasn't being picked up as a failing test.

[jennybc]

Thanks! I'm going to merge this, then make some refinements after the fact FYI.

The test for this functionality "vroom works with many connections" was incorrectly passing a character vector of paths to vroom instead of a list of connections, so this wasn't being picked up as a failing test.

It's not very apparent, but reading from *.csv.gz files actually hits the connections code path. So the pre-existing test does actually do what it claims, in a way.