plot improvements and fixes

[Tantalor93]

new features and changes:

• svg plot format is now supported and is the new default
• improved readability of all line plot plots by overlaying scatter plot more clearly

fixes:

• line graphs are now properly relative to the start of the benchmark

breaking changes in public interfaces:

• Datapoint.Duration is now time.Duration
• Benchmark.PrintReport now also accepts the time when Benchmark started

[codecov[bot]]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (62e8d35) 79.38% compared to head (1a38651) 79.44%.

Files	Patch %	Lines
cmd/plot.go	87.30%	4 Missing and 4 partials :warning:
cmd/report.go	57.14%	3 Missing :warning:
cmd/root.go	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #204 +/- ## ========================================== + Coverage 79.38% 79.44% +0.05% ========================================== Files 7 7 Lines 1140 1143 +3 ========================================== + Hits 905 908 +3 Misses 189 189 Partials 46 46 ```

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