Add linting of lines longer than 80 chars

[bradyt]

I wasn't entirely consistent everywhere, but this commit is just a sketch of adding the linter.

Would fix #161.

I could also split out the part about uri missing in widget test, and maybe script a search for typos, and add a full commit for that.

[codecov-io]

Codecov Report

Merging #162 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #162   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines         581    582    +1     
=====================================
+ Hits          581    582    +1

Impacted Files	Coverage Δ
...ges/provider/lib/src/change_notifier_provider.dart	100% <ø> (ø)	⬆️
packages/provider/lib/src/async_provider.dart	100% <ø> (ø)	⬆️
packages/provider/lib/src/provider.dart	100% <ø> (ø)	⬆️
packages/provider/lib/src/listenable_provider.dart	100% <ø> (ø)	⬆️
...es/provider/lib/src/value_listenable_provider.dart	100% <ø> (ø)	⬆️
packages/provider/lib/src/consumer.dart	100% <ø> (ø)	⬆️
packages/provider/example/lib/main.dart	100% <ø> (ø)	⬆️
packages/provider/lib/src/proxy_provider.dart	100% <ø> (ø)	⬆️
packages/provider/lib/src/delegate_widget.dart	100% <100%> (ø)	⬆️

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[rrousselGit]

Thanks!