first try at aes(link)

The idea is to write `geom_text(aes(label=some_text, link=some_url))` and then when you click on that element the browser opens another tab.

e.g. http://cbio.ensmp.fr/~thocking/H3K4me3-peaks-bases/ on the bottom in the middle of the last plot you can click on "158.0 kb on chr4" to open a new tab on that site in the genome on the genap.ca genome browser.

Works for me and tests pass -- can I get a +1?