(shell) Period not match as part of promt

[ianw]

A period in the prompt name is not matched by the shell highlighter, e.g.

aprompt $ ls -l  # matches
a-prompt $ ls -l # matches
a.prompt $ ls -l # no match

It matches other special characters such as ~[]- so I think . is a reasonable addition. I noticed this when writing some documentation that had a branch-name in the prompt, so foo-1 was working but foo-1.0 wasn't.