Added 'capthick' kwarg to errorbar()

[WeatherGod]

Exactly what it says on the label. Probably should add some examples (or modify existing ones). Tests might be nice too...

[mdboom]

Looks good. I would add or modify a regression test to use capthick so this doesn't get lost in the future. Aside from that, I think this is good to merge.

[WeatherGod]

So, looking through the tests, there is only one place where we even bother testing errorbars, and that is in the svg backend tests. Methinks it is time to re-evaluate our coverage strategy...

[mdboom]

You mean develop a coverage strategy... ;)

[mdboom]

BTW - I was suggesting adding a new test for errorbars -- and something in particular that exercises the new capthick feature.

[WeatherGod]

Almost ready with the first round of errorbar tests, slightly modified from the examples. Not exactly sure if my version of freetype (or the fonts installed) is yielding good quality baseline png images, though.

[WeatherGod]

I think this PR of mine is ready for someone to give their stamp of approval and merge it in.