ADD config option for line length

[eggnstone]

I just want more line-length configurations but those stubborn guys didn't want to change in the past 4 years, dart-lang/dart_style#833.
The .editorconfig has already been widely used and it's powerfull, so can we just use it as the default config file?
for example:

[*.dart]
max_line_length = 120

to set max line length.

Originally posted by @rainyl in #4 (comment)