[Proposal] Add support for a default-gems file

[Stratus3D]

I came across this article about the default-gems feature in rbenv: https://philna.sh/blog/2017/03/22/always-install-bundler-alongside-ruby-with-rbenv/ Might be a nice feature to add to asdf-ruby at some point. Not a high priority but definitely could be nice to have.

If anyone wants to contribute to asdf-ruby I think this might be an easy way to start. This feature should be relatively easy to implement:

• After installation of a ruby version, add some code to the install gems.
• The code should look for a file, say .default-gems in the user's home directory.
• Iterate over each line of the file, taking the text on the line and using it as the gem name in a gem install $gem command. Make sure to check the exit code and print an appropriate error message if a gem install fails. It may be possible to pass all the gems to gem install at once, I'm not familiar with the input gem install will accept.

[Stratus3D]

@thiagoa it looks like gem install isn't executed with the newly installed version of Ruby. Rather than try to set the version in a subshell it might be easier just to specify the full path to gem since we already have the path in $ASDF_INSTALL_PATH.

[thiagoa]

@Stratus3D I opened another PR to fix it up. Sorry for the hassle!

[breunigs]

I think the desired functionality is there – I tested it today and it works as one would expect. Can this be closed?

[Stratus3D]

@breunigs thanks. Yes everything should be there now.