feature: runtime config

[tlvince]

Reads in a config file (--config, default: .expose.yml) and overrides the named tuple defaults.

Example config:

TEMPLATE: tlvince
IMAGE_PATTERNS:
  - '*.png'

Note, this reuses the current config keys (UPPER_CASE). It maybe better to rename these to lower_case (or convert the config file's keys to upper case) for convenience/friendliness.