Common mp improvements, and hand-drawn looking output option

Partially in regards to issue #127, I've added the option to output hand-drawn looking versions {1, 2}. The separate `common_svg` and `common_pdf` files have been replaced by a "real" file `common.mp` which is controlled by the new `config.mp`.

`config.mp` includes tweaks that control the linewidth (which has been increased by almost 2x for the new pen), dot-width, and whether or not the hand-drawn looking output should be used (default is yes).


{1} this feature uses a third-party metapost library, see `Illustrations/README.mp-sketch` for details.

{2} it also requires metafun support -- the proper switch for `mptopdf` has already been added to the Makefile. For direct `mpost` invocation, one needs to add `--mem=metafun.mem`