Add options for justification (e.g. vjust and hjust similar to geom_text())

[rasmus87]

Is your feature request related to a problem? Please describe.
Figure edge position is figure and size dependent, since they all centre on the center of the figure.

Describe the solution you'd like
It would be nice with an option like hjust and vjust (as in geom_text()), where vjust = .5 is default figure centre, and vjust = 0 means bottom edge of the figure is exactly on the provided y, and vjust = 1, means the top edge of the figure is exactly on the provided y.

Additional context
An example figure here, where I wish to put all the phylopics bottom edges aligned with the bottom border of the plot:

Thanks for the package, and sorry if the option is available and I just couldn't find it.

[willgearty]

Hi @rasmus87, thanks for the suggestion! I'm currently working on an implementation of hjust and vjust in #102. Feel free to take a peak, although it's still not 100% done yet.