Add interactive mode for unit testing

[iamdefinitelyahuman]

What I did

Add the --interactive flag to open a console during testing. Closes #443.

How I did it

• Use the pytest_exception_interact plugin hook to enter a brownie Console on a failed test, if the --interactive flag is active.
• Add a variant of _sitebuiltins.Quitter to handle quit and exit commands without closing stdin.
• Tests and docs

How to verify it

Run tests.

[codecov-io]

Codecov Report

Merging #456 into master will decrease coverage by 0.04%.
The diff coverage is 76.66%.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
- Coverage   88.22%   88.17%   -0.05%     
==========================================
  Files          56       56              
  Lines        5911     5939      +28     
  Branches     1300     1304       +4     
==========================================
+ Hits         5215     5237      +22     
- Misses        484      488       +4     
- Partials      212      214       +2     

Impacted Files	Coverage Δ
brownie/_cli/test.py	85.00% <ø> (ø)
brownie/test/plugin.py	81.25% <33.33%> (-4.96%)	⬇️
brownie/test/managers/runner.py	88.10% <76.47%> (-1.18%)	⬇️
brownie/_cli/console.py	57.41% <90.00%> (+1.69%)	⬆️

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