v0.6.8

Changelog

bbc59ac Emit error tokens when there's a group mismatch.
a1c4eaa Add J lexer
aaa94ce svg: fixed text background rectangles
611a24c embedded font in svg (#292)
73d11b3 Clear background colour for TTY formatters.
b44683e Fix linter.
b618823 Add bibtex support.
20b4f5f Revert float pattern for C++.
6acfa13 Improve info on test regeneration (#287)
22511fb added svg formatter
0ff6248 Add multi-line comments to Nim lexer (#285)