Support slice of files and includes

[emoon]

Something that would be nice would be to allow includes(incs: &[PathBuf]) and something similar for files instead of just one include dir and one file at a time.

While it's not that hard to write

let some_include_dirs = [
  "dir0", "dir1", "dir2"
];

for dir in &some_include_dirs {
    build.include(dir);
}

If the user could supply an array/slice directly it would reduce the code a bit.

[alexcrichton]

Sounds reasonable to me to support this!

[dtolnay]

files was implemented in #205. I've opened #550 for includes after having looked for this myself.