make encoding configurable.

hi,

what's about making encoding configurable when using combine-task?

i edit my files in the eclipse IDE with UTF-8 configured, but run gradle from command line where encoding is something different then UTF-8. the resulting file end up having broken character - german öäüß in my case.

would love to see this change in your code, so i can pull the artifact from central maven repository :)

thx for your time,
yevgeniy.