Add option to hide labels on axes

This adds the `HideLabels` flag on `TicSetting` and changes `layout`, `drawXTics` and `drawYTics` to use the flag. Note that this branch will not compile if imported as `github.com/haldean/chart`, because I didn't change the imports from vdobler to haldean in imgg, txtg and svgg for easy merging. If you want to try it out, use the master branch (which does have updated import paths).