Add docker support

[Innectic]

Example of it running:

loc (lang/docker) # cargo run -- ../test
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/loc ../test`
--------------------------------------------------------------------------------
 Language             Files        Lines        Blank      Comment         Code
--------------------------------------------------------------------------------
 Docker                   1            4            1            1            2
--------------------------------------------------------------------------------
 Total                    1            4            1            1            2
--------------------------------------------------------------------------------

Contents of file:

loc (lang/docker) # cat ../test/Dockerfile
# This is a testing comment

FROM ubuntu
RUN echo "Hello world!"

Closes #95

[Innectic]

Seems these build failures aren't caused by anything I've done. Looks like all other builds have this too.

[cgag]

sorry for the crazy delay, but looks good