Add missing box drawing glyphs

[lzybkr]

A few boxing drawing glyphs seem to be missing:

• Light and heavy dashed lines (U+2504 through U+250B)
• Character cell arcs (U+256D through U+2570)

See sharkdp/hexyl#17 for a scenario where a light dashed line was used in a handy hex dumping tool.

[tonsky]

Thanks, should be no problem adding those