Add rescale, colormap, expression tiler parameters

[lucyhutcheson]

Related Issue(s):

• Gracefully handle case where titiler is not configured #8
• If the assets parameter is not defined for a collection, don't try to show the tile layer #35

Proposed Changes:

1. Create new REACT_APP_MOSAIC_TILER_PARAMS env variable
2. Add support for rescale, colormap_name, expression tiler parameters

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[philvarner]

looks good, and I tested it works. Minor refactoring changes requested in comments.

[philvarner]

inline