export: add option to create poster static map

I added this functionnality as I needed it myself. It doesn't really address any existing issue, hence this pull request.
I intend to improve upon it, but getting your attention on it would help in orienting my efforts in the right direction.
What it does is add 2 params to the command line export command (--pages_x and --pages_y) to define over how many pages across and down you want your static export broken down. Once printed, the pages can be assembled to produce a poster. It should also take into account #1587 to make more sense.