[DvBree]

Description

• New feature: it is now possible to put environment variables in the config as {{env.NAME}}. This feature was also present in t_rex.
• Small change for config w.r.t earlier versions of texel mvt: now only a single tileset can be specified (which is sufficient for PDOK applications).
• Updated README with config for generating tiles.

Type of change

• New feature

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR