(shell) Period not match as part of promt

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

```console
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.