v2.8.0

UnicodePlots v2.8.0

Diff since v2.7.0

Merged pull requests:

• Fix :both on arrow, improve coverage (#214) (@t-bltg)
• Fix canvas example (randn -> rand), docs (#215) (@t-bltg)
• Unicodeplots docs (#216) (@t-bltg)
• Add surfaceplot, isosurface (support 3d plots) (#217) (@t-bltg)
• Update docs for surfaceplot, isosurface (#218) (@t-bltg)
• Minor fixes (#220) (@t-bltg)