Config via JSON needs documentation

[eggnstone]

@staginghub reported in eggnstone/DartFormatJetbrainsPlugin#20:
"I've also tried to use dart_format with json, but i'm getting an error. Theres no documentation on the command line json format.

[eggnstone]

@staginghub
Please see https://github.com/eggnstone/dart_format/blob/master/example/main.dart
Hope this helps a little.

[rainyl]

Great poject, 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.

[eggnstone]

@rainyl Thanks! As for your suggestion I created a separate issue: #5