add tags to docs view, dag view filters

[drewbanin]

Closes #6

This PR replaces the node_type filter with tags, as there isn't generally a need to filter by node types in the graph viz.

I needed to re-implement some more of the dbt graph selection code in JS -- I think we'll need to figure out better ways of doing this if/when graph selection gets more complex.

Relevant tags PR: dbt-labs/dbt-core#1014

Test:

• model view shows tags for a model, or None
• DAG view is filterable by tags in a dropdown filter
• --models and --exclude accept tag: and fqn: filter strings
• dropdown tag filters play nice with fqn/tag selection using --models and --exclude

[drewbanin]

Screenshots!

[cmcarthur]

@drewbanin hey sorry I missed this. can you throw a class="meta" or <em> on untagged to differentiate it? then 👍