Add ability for yetibot to delete its own message triggered by delete…

[devth]

… emoji

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.06 :warning:

Comparison is base (3bd2628) 62.13% compared to head (fb2a401) 62.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #222 +/- ## ========================================== - Coverage 62.13% 62.07% -0.06% ========================================== Files 96 96 Lines 4183 4187 +4 Branches 231 231 ========================================== Hits 2599 2599 - Misses 1353 1357 +4 Partials 231 231 ``` | [Impacted Files](https://app.codecov.io/gh/yetibot/core/pull/222?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/adapters/slack.clj](https://app.codecov.io/gh/yetibot/core/pull/222?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yetibot#diff-c3JjL3lldGlib3QvY29yZS9hZGFwdGVycy9zbGFjay5jbGo=) | `52.99% <0.00%> (-0.62%)` | :arrow_down: |

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