MarkdownBear: Set max_line_length default to None

[jayvdb]

The introduction of max_line_length with a default of 80
was a breaking change as previously line length was not
constrained.

This constraint required the use of remark-lint, which
caused MarkdownBear to remove all content from files
if remark-lint could not be found in NODE_PATH.

The default is now again to not check line length,
and not use remark-lint unless the .coafile explicitly
requests a max_line_length.

Fixes #1581

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[sils]

ack 0cf749b

[jayvdb]

@rultor merge

[rultor]

@rultor merge

@jayvdb OK, I'll try to merge now. You can check the progress of the merge here

[rultor]

@rultor merge

@jayvdb Done! FYI, the full log is here (took me 2min)