tell: avoid errors produced by empty tellee

[SnoopJ]

Description

This changeset is an alternative to #2582 that transforms the tellee as early as possible.

The use of lstrip("@") in this patch means that an arbitrary number of prefixing @s will be removed, so that @@Someone is transformed into Someone where it would previously have been transformed to @Someone, but since this is an edge case to begin with, I don't think it's any problem.

Longer term, we'd like to have better awareness of if tellee is a valid nick or not. I've filed #2583 for this purpose.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make lint and make test)
• I have tested the functionality of the things this change touches

[dgw]

Note to self: Manually close #2582 when merging, since GH artificially doesn't allow merging a PR to auto-close another PR. (I say "artificially" because you can auto-close issues, and "all PRs are issues but not all issues are PRs" according to the GH API.)

[Exirel]

I'd add a few @plugin.examples for the new cases. Like:

@plugin.example('.ask @', 'ask whom?')
@plugin.example('.tell @', 'tell whom?')
@plugin.example('.tell @Exirel:', 'tell Exirel what?')

That way we can test that the command removes some characters from the tellee.