Introduce (repeatable) --require option

This eliminates all optional dependencies in favor of autodetection (via
tilelive-modules) or requiring them explicitly (and detecting their
instantiation pattern) when starting tessera:

    tessera -r mbtiles -r tilelive-carto tessera.json

TESSERA_OPTS can contain partial options to avoid having to repeat
oneself:

    export TESSERA_OPTS="-r mbtiles"
    tessera -r tilelive-carto tessera.json

Refs #4