Labeling polish

[nvkelso]

• [x] trails – vector tiles currently remove trail labels at zoom 11, 12, and 13. Hopefully a dev build will restore names soon, via https://github.com/mapzen/vector-datasource/issues/596. Only so much we can do without Tangram curved text labels, though.
• [x] roads - probably hide exits for a few zooms
• [x] places - halo to land color
• [x] landuse - show more park labels earlier? relax zoom / area filters?
• [x] pois - Related: #1 and #5. Most POIs not related to outdoor should be zoom 16+ only – and some earlier ones like schools maybe show later too? Watch out for kind:residential generic sprites!
• [x] water - blue label fill. show more water labels earlier? relax zoom / area filters?

[nvkelso]

Related: https://github.com/tangrams/outdoor-style/issues/5

[nvkelso]

Related: https://github.com/tangrams/outdoor-style/issues/39

[nvkelso]

Don't see highway exits at all. Sometimes trail names are in green & grey.

[nvkelso]

Fixed highway exits in 4eee131135fe1383ec772a75ec4e4b47d4317758.