adds "exclude" knob in configuration

[chavacava]

Closes #244 by adding an exclude field in the configuration. This field accepts an array of paths to exclude from the analysis. Path syntax is the same of that of the -exclude command line flag; for example the configuration

ignoreGeneratedHeader = false
severity = "warning"
confidence = 0.8
errorCode = 0
warningCode = 0
exclude = [
  "rule/...",
  "testdata/..."
]

will exclude the same files as if we execute revive with the following command line

revive -exclude rule/... -exclude testdata/... 

Excluding paths through command line flags overrides the configuration.