add warning if follower is behind

[mehtaphysical]

add a warning to pg:unfollow if the follower is behind

[codecov-io]

Codecov Report

Merging #187 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #187      +/-   ##
=========================================
+ Coverage   95.39%   95.4%   +0.01%     
=========================================
  Files          54      54              
  Lines        1347    1350       +3     
=========================================
+ Hits         1285    1288       +3     
  Misses         62      62

Impacted Files	Coverage Δ
commands/unfollow.js	100% <100%> (ø)	⬆️

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[msakrejda]

The warning is not terribly actionable and comes after an irreversible operation... What's our goal here? Can/should we warn before the action? Can we link to more information on follower lag?