Minor updates to ggcorr and ggnet

Hi Barret,

Here are some minor updates for `ggcorr` and `ggnet`:

* `ggcorr`: argument to print correlation coefficients on top of colored tiles
* `ggnet`: options to label mid-edges and pass complex graphing parameters

I've updated the [demo](https://github.com/briatte/ggcorr) [pages](https://github.com/briatte/ggnet) accordingly.

Cheers (and a happy new year!),

François