Add --prefix option for use in PATH

[bsander]

If I want to access one of the elasticsearch binaries (like plugin), I need to directly access them. It would be great to have a --prefix switch like brew so I could add something like the following line to my PATH variable for easy access:

PATH="$(esvm --prefix)/bin:$PATH"

Thanks for a great tool!

[spalger]

Interesting, I like the idea