Create Base Stat Monitor class to help creating custom monitors

[rennerocha]

Solves #321

This PR includes a easier way to create custom monitors based on simple comparisons between a job stat and a threshold. We are talking usually about numerical values, but for EQUAL and NOTEQUAL, this can be strings as well.

A monitor that checks if a stat called test_stat is greater than or equal to a value defined in a project setting called TEST_STAT_THRESHOLDcan be defined as follows:

    class TestStatMonitor(BaseStatMonitor):
        stat_name = "test_stat"
        threshold_setting = "TEST_STAT_THRESHOLD"
        assert_type = AssertionType.GTE

The threshold value can also be defined by a method, so we are able to create more complex rules to define what the threshold should be. Some ideas that can be accomplished using that method:

• Compare the number of items returned with the number of the latest job
• Define the threshold as a percentage of the number of input URLs against the number of requests
• etc

    class TestStatMonitor(BaseStatMonitor):
        stat_name = "test_stat"
        assert_type = AssertionType.GTE

        def get_threshold(self):
            # Do something to get the value
            return threshold

We still need to add the docs for this new feature and rewrite most of the built-in monitors to use this new structure.

[codecov]

Codecov Report

Merging #325 (dac6ebb) into master (d58090d) will increase coverage by 0.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
+ Coverage   73.54%   73.92%   +0.37%     
==========================================
  Files          68       68              
  Lines        2967     2991      +24     
  Branches      335      451     +116     
==========================================
+ Hits         2182     2211      +29     
+ Misses        723      718       -5     
  Partials       62       62              

Impacted Files	Coverage Δ
spidermon/contrib/scrapy/monitors.py	98.15% <100.00%> (+0.31%)	⬆️
spidermon/results/monitor.py	78.51% <100.00%> (+2.47%)	⬆️
spidermon/contrib/scrapy/runners.py	88.76% <0.00%> (+2.24%)	⬆️

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[further-reading]

LGTM overall. I have a suggestion on how to make it a little clearer from a user perspective but it is still fine as is!

[further-reading]

LGTM :)

[rennerocha]

Added the missing docs. This PR is now ready for real review and when approved, to be merged :-)

[ogiaquino]

LGTM