Make autoreload optional and disabled by default

[jacobtomlinson]

Autoreloading isn't working perfectly at the moment.

So for now I'm putting it behind a feature gate. To enable it you need to add autoreload: true to your config.

[codecov]

Codecov Report

Merging #1482 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1482   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files          52       52           
  Lines        3281     3283    +2     
=======================================
+ Hits         3278     3280    +2     
  Misses          3        3           

Impacted Files	Coverage Δ
opsdroid/configuration/validation.py	100.00% <ø> (ø)
opsdroid/core.py	100.00% <100.00%> (ø)

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