Add support to parse a filelist specified within a Bender.yml

[mrogenmoserTT]

No description provided.

[phsauter]

My Rust is still bad but a few things I think are currently not handled:

1. It should split on whitespace not on lines since there can be multiple things (eg incdirs) on one line
2. I think it currently treats empty lines as a file path, probably bad but point 1 would fix this anyway
3. Paths should probably be canonicalized and then made relative to the base-dir just to be sure
4. +define and +incdir syntax can sometimes be chained as in +incdir+dir1+dir2
5. Sometimes you can generate file lists relative to some dir or relative to the file itself, do we want to support that?
6. We should strip comments in the file list (after trim and split any string that starts with // or #

[mrogenmoserTT]

The most recent push should address 1,2,4, and 6. For the paths, right now this only supports files relative to the file list file path, as making these file lists relative to yet another directory would require more information to be passed. Properly relativizing the path still needs to be addressed.