Ensure that equals-options get parsed properly

Options such as `--userimg=<path>` should be parsed as an option with an equals in its name (`userimg=`) and without the path argument in `Option.name`.

This solves https://github.com/staticfloat/homebrew-julia/pull/128, where passing `--userimg=/path/to/userimg.jl` would not cause a build from source, but would rather pour a bottle.