Add basic pre-commit config

[hugovk]

Fixes #149.
Fixes #150.

More can be added as required (see https://pre-commit.com/hooks.html), but this adds a basic pre-commit config file to stop the check erroring PRs (e.g. #157):

[codecov]

Codecov Report

Merging #158 (ce3601d) into master (48ce16b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #158   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          764       764           
  Branches       157       157           
=========================================
  Hits           764       764           

Impacted Files	Coverage Δ
jsonmodels/builders.py	100.00% <100.00%> (ø)
jsonmodels/fields.py	100.00% <100.00%> (ø)
jsonmodels/models.py	100.00% <100.00%> (ø)
jsonmodels/parsers.py	100.00% <100.00%> (ø)
jsonmodels/utilities.py	100.00% <100.00%> (ø)
jsonmodels/validators.py	100.00% <100.00%> (ø)

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[hugovk]

Updated to add isort and Black, for #149 and #150.