Slider: Allow styling of track tick colour

[WillsB3]

Currently, as far as I can see, tick marks are always rendered in black. Because the ticks are drawn via canvas they are not customisable with CSS.

We have a use case where a white slider is rendered on a dark background. This makes the tick marks difficult to see (infact, it just looks like there are gaps in the sliders track).

[ThomasBurleson]

The ticks and track should be part of the theme solution. Thank you for the alert.

[ajoslin]

These are now themable. The background-color of the '.md-track-ticks' element is read with getComputedStyle().

[WillsB3]

Excellent! Thanks :)