Add default diff support

[CosmicHorrorDev]

This adds support for the default diff syntax. It looks like this is missing from pygments too, so I'll have to open a PR with them tomorrow

I didn't know what token type was appropriate for the separator, so I went with GenericStrong, but feel free to suggest something else

Demo

default.diff

1,2d0
< A
< A
4c2
< C
---
> F
5a4
> E

Generated with

old.txt

A
A
B
C
D

new.txt

B
F
D
E

$ diff old.txt new.txt > default.diff

Results

chroma --format="tokens" default.diff

Before

&Token{Text, "1,2d0\n< A\n< A\n4c2\n< C\n"}
&Token{GenericDeleted, "---\n"}
&Token{Text, "> F\n5a4\n> E\n"}

After

&Token{GenericSubheading, "1,2d0\n"}
&Token{GenericDeleted, "< A\n< A\n"}
&Token{GenericSubheading, "4c2\n"}
&Token{GenericDeleted, "< C\n"}
&Token{GenericStrong, "---\n"}
&Token{GenericInserted, "> F\n"}
&Token{GenericSubheading, "5a4\n"}
&Token{GenericInserted, "> E\n"}

[alecthomas]

Nice, thanks!