Add slack emoji command

[codecov[bot]]

Codecov Report

Patch coverage: 27.27% and project coverage change: -0.10 :warning:

Comparison is base (85844ca) 62.22% compared to head (6ec6472) 62.13%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #221 +/- ## ========================================== - Coverage 62.22% 62.13% -0.10% ========================================== Files 95 96 +1 Lines 4172 4183 +11 Branches 231 231 ========================================== + Hits 2596 2599 +3 - Misses 1345 1353 +8 Partials 231 231 ``` | [Impacted Files](https://app.codecov.io/gh/yetibot/core/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yetibot) | Coverage Δ | | |---|---|---| | [src/yetibot/core/commands/slack.clj](https://app.codecov.io/gh/yetibot/core/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yetibot#diff-c3JjL3lldGlib3QvY29yZS9jb21tYW5kcy9zbGFjay5jbGo=) | `27.27% <27.27%> (ø)` | |

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