fix(format_value) handle unset keys

This revises the previous change to check for a variety of keys that
may or may not be set when a value is passed to format_value() to avoid
throwing exceptions (hopefully).

fix(format_value) handle unset value

Zelda was throwing an error when (I believe) a field was saved with no
value, because it was trying to format a value that didn't exist. This
is behavior we actually want to allow (i.e. the field is not required)
so this fix simply returns the content passed for formatting if the
`value` portion of the array is not set.

fix(checks): add some stuff to deal w/ unexpected field values

cache post results for better perf

return ALL posts

Improve appearance

refactor 'external' to 'uri' to be more flexible

Allow setting return format

Update composer.json for distribution

test external for locality; save as relative if so