mvn: absolute raml input files should be allowed

[adrobisch]

Currently only the classpath is scanned for the input file. It should be possible to provide absolute paths.

[gino0631]

That would be good. I have managed to specify a RAML file in src/raml by using <raml>../../src/raml/api.raml</raml> configuration option (after looking at the code), but it's more like a workaround.