Add basic constraints

[jacobtomlinson]

Description

This PR adds some constraint decorators for use when writing skills. These allow you to restrict when skills are run even if they are matched. The initial constraints I've added are constrain_rooms, constrain_users and constrain_connectors which allow you to restrict a skill to a set of rooms, users or connectors respectively.

This is the precursor to adding context for #156.

Status

READY

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Added new tests and run locally.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #745 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #745   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          27     28    +1     
  Lines        1804   1843   +39     
=====================================
+ Hits         1804   1843   +39

Impacted Files	Coverage Δ
opsdroid/parsers/witai.py	100% <100%> (ø)	⬆️
opsdroid/matchers.py	100% <100%> (ø)	⬆️
opsdroid/parsers/regex.py	100% <100%> (ø)	⬆️
opsdroid/parsers/luisai.py	100% <100%> (ø)	⬆️
opsdroid/constraints.py	100% <100%> (ø)
opsdroid/parsers/recastai.py	100% <100%> (ø)	⬆️
opsdroid/helper.py	100% <100%> (ø)	⬆️
opsdroid/parsers/dialogflow.py	100% <100%> (ø)	⬆️
opsdroid/parsers/rasanlu.py	100% <100%> (ø)	⬆️
opsdroid/core.py	100% <100%> (ø)	⬆️
... and 1 more

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[FabioRosado]

This looks great, I was thinking how to specify a certain user to interact with opsdroid as we have mentioned this from time to time on gitter/matrix. I've just updated this branch and will merge the PR as soon as travis is happy