Add support for http notifications

[dataoscar]

Add an additional notifier that supports http requests using
GET, POST, and PUT methods.

This also adds support for declaring additional
notifications at the backup configuration. This is helpful
when HTTP notifications would be different per backup job and
would otherwise use different URLs.

Fixes #51, closes #55

[codecov]

Codecov Report

Merging #56 (6277f08) into master (ef85279) will increase coverage by 2.55%.
The diff coverage is 58.00%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   40.36%   42.91%   +2.55%     
==========================================
  Files          12       13       +1     
  Lines         934      932       -2     
==========================================
+ Hits          377      400      +23     
+ Misses        515      479      -36     
- Partials       42       53      +11     

Impacted Files	Coverage Δ
configBackup.go	46.71% <23.63%> (-3.66%)	⬇️
configGlobal.go	75.82% <54.83%> (+0.82%)	⬆️
http_notifier.go	87.50% <87.50%> (ø)
emailNotifier.go	42.30% <100.00%> (+2.91%)	⬆️
timeutils.go	72.88% <0.00%> (-10.99%)	⬇️
executor.go	63.63% <0.00%> (-5.60%)	⬇️
utilities.go	68.42% <0.00%> (-3.01%)	⬇️
checkpoint.go	81.25% <0.00%> (-1.61%)	⬇️
... and 7 more

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