Add support for colored legend

[EwanMe]

Legend can be added to a graph by including the option asciigraph.LegendTexts. This option is a list of strings making up the legend texts, whose indexes correspond to the series in the graph. Each legend item has a colored box matching the series it is mean to explain.

[coveralls]

Pull Request Test Coverage Report for Build 8411757830

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 35 of 37 (94.59%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 95.89%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
legend.go	28	30	93.33%

Totals
Change from base Build 5568018373:	-0.1%
Covered Lines:	350
Relevant Lines:	365

---

💛 - Coveralls

[guptarohit]

This is looking great @EwanMe ! ⭐ thanks

I'll add test for this and merge it very soon.

[EwanMe]

@guptarohit Cheers!