Add parameter for slack aggregation table width

[vladimir-ilyashenko]

No description provided.

[nsano-rururu]

Conflicting file error.
This is an example of the command to be resolved.

git remote add upstream git://github.com/Yelp/elastalert.git
git fetch upstream

git merge upstream/master
git push origin add-parameter-for-slack-aggregation-table-width

[vladimir-ilyashenko]

@nsano-rururu conflict fixed