add a sourceDirectory option

[zosrothko]

Hi

When using a code generator of c# code like JavaCC under maven, the c# is by default located in target/generated-sources/javacc, but the plugin does not seem to take this directory into account. So could the dotnet-maven-plugin check for c# code in the so called target directory or could be an options added to specify where are the c# code like sourceDirectory for exmaple.