Formatter: Allow customization of IDs

[clarisma]

For formats that include a feature ID (currently one GeoJSON), enable the ability to specify a template for IDs (by default, each ID is a string consisting of the uppercase type letter and the feature's ID, e.g. N123). Users may prefer other formats, such as node/123 or a plain number (may need a transformation to ensure uniqueness across OSM types) -- see #35.

Proposed:

• id attribute: a template similar to f-strings, or a constant that refers to a predefined ID scheme (see proposed approach for the GOL tool: https://docs.geodesk.com/gol/query#option-f-id