Restrict characters in package 'tags'

[emdoyle]

While planning work on Issue #49 , I realized that we should restrict and validate package 'tags'. This makes TOML support much more straightforward, and will probably be useful in general if we end up using 'tags' as input to other commands or functionality.

We should use a regex like the following:
^[a-zA-Z0-9]([a-zA-Z0-9_-]*[a-zA-Z0-9])?$
which would allow only alphanumeric characters, underscores, and dashes, while also always starting and ending with an alphanumeric character.

[emdoyle]

Closing this in favor of turning 'constraints' into a list of dictionaries, and allowing arbitrary strings in a 'tags' key.

tach init already uses dots by default to tag nested packages, and this is intuitive behavior we should keep. We also use colons in our own 'tach' installation.