Add keep_path global variable (#759)

[tlienart]

in config.md users can now define keep_path as a list of things they don't want Franklin to modify the path to (see #759 for context)

@def keep_path = ["foo.html", "bar/baz/"]

what that will do is:

• the file <folder>/foo.html will be copied to <folder>/__site/foo.html (no injection of index.html)
• the files in <folder>/bar/baz/ will be copied to <folder>/__site/bar/baz/ without injection of index.html

this also works for .md files btw so

• the file <folder>/foo.md with this setting will be made available at <domain>/foo.html and not at the default <domain>/foo/index.html (ie <domain>/foo/)

@ViralBShah I'll merge this in master now, and will release it once you've had a chance to play with it; hopefully it should just work.