mel list: support --format for flexible printing
mel list: support --format for flexible printing

Make it easier to do advanced queries on a list of moles by allowing
customisation of the output format. Expose a new key 'lastmicro'
initially.

In later work we may add e.g. 'lastmicro_age_days', 'num_micro', etc.

Test Plan:
$ ./meta/run_tests.sh

In mel data dir, make sure output as expected:
$ mel list
$ mel list --format '{relpath}'
$ mel list --format '{lastmicro} {relpath}'
$ mel list --format '{lastmicro} {relpath}' --sort lastmicro

aevri/mel,aevri/mel