Add support for linting unsaved files

[Gozala]

I often times create a new buffer and start sketching things up, which is especially useful for reporting issues to a flow project. Unfortunately such workflow is not supported and I tend to use https://flowtype.org/try/ instead. I would very much prefer to use my configured editor instead of https://flowtype.org/try/

[Gozala]

I looked into this as I wanted write a patch to fix this, but as it turns out linter does not event trigger providers for buffers that have no files associated with them :( So it's blocked by steelbrain/linter#1235

[Arcanemagus]

Closing this here, if and when linter adds support for that this can be revisited.