Allow passing multiple file names to graph, to get multiple lines in HTML

[devdanzin]

This PR addresses a TODO comment in graph.py: "Add multiple lines for multiple files". It turns path from a single argument into a variadic argument and plots lines for each file passed (and for those files in any directories passed too). If --aggregate is used, it aggregates the data for any directories passed, but also displays individual lines for any files passed (we can't easily aggregate those, as the aggregated data is recorded on build).

 wily graph .\src\wily\cache.py .\src\wily\state.py .\src\wily\commands\report.py .\src\wily\commands\diff.py .\src\wily\commands\index.py -m loc --all

 wily graph .\src\wily\cache.py .\src\wily\state.py .\src\wily\commands\report.py .\src\wily\commands\diff.py .\src\wily\commands\index.py .\src\wily\operators\ -m loc --all

wily graph .\src\wily\cache.py .\src\wily\state.py .\src\wily\commands\report.py .\src\wily\commands\diff.py .\src\wily\commands\index.py .\src --aggregate -m loc --all

To achieve this, an incompatible change is needed: metrics, which previously was a variadic argument, now is a required option instead (which means it's always necessary to pass -m metric_name). To pass two metrics, users need to provide them as comma-separated values (same as in diff). Another way to go about this would be to add an explicit z_axis option, so when users want a metric in Z axis they provide it as this option.

Listing multiple file names in the graphs title simply doesn't work after a handful of files. So this PR avoids printing file names altogether when more than one path is supplied.

I like the comma-separated option because it matches diff and also because I'd like to introduce a new command using the same solution (it takes arbitrary number of paths and metrics, so it'll need a way to pass multiple metrics as non-variadic argument or option).

[codecov-commenter]

Codecov Report

Merging #206 (3a62254) into master (acf39d2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   95.46%   95.48%   +0.01%     
==========================================
  Files          26       26              
  Lines        1346     1351       +5     
  Branches      287      287              
==========================================
+ Hits         1285     1290       +5     
  Misses         37       37              
  Partials       24       24              

Files Changed	Coverage Δ
src/wily/__main__.py	95.69% <100.00%> (ø)
src/wily/commands/graph.py	100.00% <100.00%> (ø)

[tonybaloney]

Nice! Great improvement