Add pinnacle#dump(), counterpart to pinnacle#highlight()

Add pinnacle#dump(), counterpart to pinnacle#highlight()

Oh what fun it is to program in Vimscript. That `filter` call needs to
use string comparison because "any string" == 0 despite the fact that
strings are truthy... The `filter` expression must return 0 or not-0.

Given that pinnacle#dump() will return a prefix key if applicable (term,
cterm or guiterm), make sure pinnacle#highlight() can consume
dictionaries with those keys.