Allow Collection#join to be used without a block

[george-carlin]

Most of the time when I use Collection#join, I don't actually care about calling custom methods etc., I just want to stick some HTML in between each rendered cell. But #join insists on taking a block, so I have to do something like this:

my_collection.join('<hr>') { |cell| cell }

This PR makes the block optional, so I can just do it like this:

my_collection.join('<hr>')

[george-carlin]

If you approve this then I updated the docs too at trailblazer/trailblazer.github.io#91

[george-carlin]

Can we pretty please get a release that includes this feature? :)

[apotonick]

cells$ gem push cells-4.1.7.gem 
Pushing gem to https://rubygems.org...
Successfully registered gem: cells (4.1.7)

[george-carlin]

Thanks! :)