Allow Running Custom Commands Per "Lifecycle"

[jbolda]

This is a tracking issue for the objectives required for the PR as discussed in https://github.com/atlassian/changesets/issues/310.

TODO:

• [x] Allow config.json to accept package options where the user can specify custom commands to be run during each "lifecycle"
  • This will allow someone to specify a script listed in their package.json if they want to add functionality to run on top of the standard functionality
  • This will also allow someone to use a completely custom command in the case of languages other than javascript. Someone can specify a command that runs and increments a package version in the language of their choosing, e.g. cargo release.
• [x] Pass the config options with the existing hardcoded defaults as an optional default.
  • This will maintain backwards compatibility and then allow someone to specify a custom command pulled from the previous bullet point.
• [ ] Update the get-workspaces package to look for packages that created when using languages other than javascript. (Likely the greatest effort.) This is now actually in a separate repository.
  • Creating a plugin system is the easy answer, but we are looking to start with built in and hopefully make it extensible enough that we don't need a plugin system.

[jbolda]

@ryaninvents I have been tracking notes here as I've been working on the PRs. I presume the tests could use a good bit of love (as I can't really run them). I think I have the structure mostly roughed in, but I presume the tests will suss out the edge cases.

Thanks again for the help!