Implement file list include / exclude & force_include sections

[wolfv]

In conda-build, one can do split packages by specifying a "files" field. In the files field, one can have a list of "globs" to include into the package.

However, there are some shortcomings: it's not easy to exclude files, and the file globs actually operate on the entire prefix and not just the file-diff.

I want to implement new sub-keys: include, exclude and force_include.

include operates on the newly added files and selects them by glob. After that, files in exclude are subtracted from the included files. with force_include, one can select files from the prefix that do not need to come from the newly added files (ie. the entire prefix). That should be avoided in nearly 100% of the time.

[wolfv]

This is done in master :)