Add a simple stop-words bot

[alek-sys]

It turned out to be quite different to integrate it with wtf bot, so it was worthwhile extracting into own thing.

[coveralls]

Pull Request Test Coverage Report for Build 22035658461

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 84 of 85 (98.82%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 75.301%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app/main.go	0	1	0.0%

Totals
Change from base Build 14460942638:	0.6%
Covered Lines:	2692
Relevant Lines:	3575

---

💛 - Coveralls

[umputun]

couple of issues:

1. \b doesn't work with Cyrillic in Go's RE2 - saynomore.go:59 uses \b for word boundaries, but Go's regexp defines \b via \w which only matches [0-9A-Za-z_]. the bot will never match any of its default Russian stop words. tests pass because they use ASCII words ("badword", "rude"), masking the bug. fix options: use (?:^|\s) / (?:\s|$) instead of \b, or just use strings.Contains on lowercased text since QuoteMeta is already applied (no regex features needed)

2. panic on empty responses - saynomore.go:106 calls s.rand(int64(len(matched.responses))), if a category has empty Responses slice, rand.Int63n(0) panics. a simple check in NewSayNoMore would prevent it

3. no test for NewDefaultSayNoMore with actual Cyrillic input - a single test with a real Russian phrase would have caught issue Cover reporter package with unit tests #1

otherwise the design is clean, follows the WTF bot pattern well, and the code is readable

[umputun]

lgtm, fixes look good