Improved --gas report

[matnad]

What I did

grouped by contract
sorted by average gas
padded for table-like view

Gas Profile:
BrownieTester <Contract>
   ├─ constructor   -  avg: 1099591  low: 1099591  high: 1099591
   ├─ sendEth       -  avg:   23017  low:   23017  high:   23017
   └─ doNothing     -  avg:   21437  low:   21437  high:   21437
EVMTester <Contract>
   ├─ constructor   -  avg:  711445  low:  711445  high:  711445
   └─ modulusByZero -  avg:   21658  low:   21658  high:   21658

Closes #536

How I did it

String formatting

How to verify it

python -m pytest tests/test/plugin/test_output.py::test_print_gas --target plugin -n 0 -s --no-cov

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation
• I have added an entry to the changelog

[codecov-commenter]

Codecov Report

Merging #543 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #543      +/-   ##
==========================================
+ Coverage   88.40%   88.47%   +0.07%     
==========================================
  Files          56       56              
  Lines        6259     6276      +17     
  Branches     1381     1386       +5     
==========================================
+ Hits         5533     5553      +20     
+ Misses        496      494       -2     
+ Partials      230      229       -1     

Impacted Files	Coverage Δ
brownie/test/output.py	92.51% <100.00%> (+0.97%)	⬆️
brownie/test/coverage.py	100.00% <0.00%> (+6.66%)	⬆️

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[iamdefinitelyahuman]

Looks good! Would you mind rebasing off master rather than merging though?