Add ScanFatalErrors command

[TomasVotruba]

Closes #2388

Useful for creating param/return type completion based on your specific code

vendor/bin/rector scan-fatal-errors src

Then run Rector to complete param/return types:

vendor/bin/rector process src --config rector-types.yaml

[TomasVotruba]

It requires more work to detect multiple fatal errors in single file.
So far it support errors on invalid parent type (unlimited) and invalid return type (single per file) - in violation with parent one.

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Now as a part of the build:

https://travis-ci.org/rectorphp/rector/jobs/629125978#L406