Make environment creators more extensible / scriptable

[storborg]

Not sure of good ways to do this, but it should be easy to add a new creator.

[storborg]

Here's my thinking for how to implement this in a way that doesn't involve managing massive namespaces or making things really hard to share:

1. Split out existing 'creators' into folder built into dram, with a shell script per creator named like macports.sh, plain.sh, etc.
2. Allow the user to augment or override the built-in creators with a directory like ~/.dramrc/creators.
3. Mayyybe also allow specifying a shell script as a path or an http(s) URL on the command line, so you could do something like dram create -t https://www.example.com/setup-environment.sh my-dram-name.