(PDK-1416) Support Gemfile source option

[seanmil]

Add the ability to specify a per-gem source where needed.

[scotje]

@seanmil Thanks for the contribution! Would you mind adding something about this option to the README? Once we have that I think this will be ready to merge.

[seanmil]

I don't mind adding something about it, but none of the other gem options are documented anywhere in the PDK that I could see. I would have expected them around here: https://github.com/puppetlabs/pdk-templates#gemfile

How would you think that should look? Another table in the Gemfile section or something else?

[scotje]

Indeed the existing options don't appear to be documented at all :)

I agree that the section you linked would be the best place for it. I think if we add an example below the existing table and then a new table that lists the options for each entry that would be good. If you want I can work on adding docs for the existing items but if you want to take a pass at it please feel free!

[seanmil]

I've added documentation for all of the options in a gem item, an example in the examples area, and clarified a couple of other points around specifying custom gems which I felt could have been a little clearer. Let me know. Thanks!

[scotje]

Looks good @seanmil, I added one suggestion to fix a typo and add a link to Rubygems docs on version ranges. Looks like CI is failing but I haven't had a chance to look at that yet.

[scotje]

Thanks for your contribution to PDK @seanmil!