support `-html` flag to render output as HTML

[mdb]

This addresses issue #69 to support HTML output.

This also...

• updates the README usage docs
• validates that multiple, conflicting format flags are not provided
• fixes a bug in the tests to properly validated expected errors invoking tf-summarize
• improves the tf-summarize tests to exercise the tool's ability to summarize changes to outputs (as well as resources)
• improves make help to output documentation for all Makefile targets
• improve test accuracy via -count flag, protecting against false positives in go test runs

Seed plant for the future: Ultimately, it might be preferable to support a single -format="json|md|html|table" flag, though I was hesitant to do so in this PR and break tf-summarize's existing usage in a non-backwards-compatible fashion.

[mdb]

👋 Hi @dineshba - I'm curious to hear your thoughts about this PR!