Accept user flags and arguments

[montanaflynn]

The README is a lie, the real usage is as follows:

go run main.go creates a file called meme.png in the current working directory.

We want it to be like the README:

./mgen --meme fry --text "not sure if magick or golang" ./path/to/file.png

Will want to have docopt style short and long flags but the basic usage is thus:

Usage for meme-generator:
  meme-generator [options] ./path/to/file.png

[montanaflynn]

Added basic parsing in 6a5147c but it can still be cleaned up a bit around the edges.