Add museum icon and brown "attraction" POI color

[wmisener]

Fixes #820, and makes progress on #692 (well, actually museums aren't explicitly listed in #692, but they probably should be). I've used a modified version of the columned, neoclassical building I originally proposed for town halls in #794, but without the flag:

This icon specifically evokes the large, established institutions of big cities, but I think the symbol is recognizable enough to represent museums of diverse sizes and settings.

This is the first POI in the "attraction" category, so the PR also adds and documents a new POI color, "attraction brown". Brown was suggested in #128, and it seems reasonable for attractions based on American usage, such as MUTCD signage. As @1ec5 pointed out on Slack, care may be needed when indigenous land boundaries are added (#105) to make sure the colors in the style clearly communicate their intended meaning. I've reused the brown color defined for highways, hex #693F23, but would be amenable to alterations if someone has a strong opinion.

Samples:
National Mall (localhost):

Woods Hole, MA (localhost):

San Francisco, CA (localhost):

Chicago, IL (localhost):

Note that the Field Museum (tourism=museum) is rendered in this PR, but the Shedd Aquarium (tourism=aquarium OMT class=aquarium, subclass=aquarium) and the Adler Planetarium (amenity=planetarium, which doesn't appear to be in OMT, although I think this might be a tagging error) are not.

[ZeLonewolf]

Visual inspection and code looks good to me. I'm glad we could re-use the work on this. Like to get someone on one of the fancier displays to make sure it looks good at all resolutions.

[claysmalley]

Looks good to me.