Allow user-defined output formats

[luhring]

It would be great to allow user-specified output formats (such as by templating) so that Grype could support arbitrary text formats.

This would mean that Grype could immediately support downstream consumers' needs, even if no presenter specific to that consumer's input format had been developed and incorporated into the Grype code.

Notably, this would especially help with Grype integrations into other systems, which would enable more native workflows that actively depend on Grype.

(This is also related to anchore/syft#152)