Test and document modular features

[breezykermo]

There are four features in timemap that can be disabled or enabled:

• tags
• search
• sites
• sources

Each of these has an enabling flag in features of config.js. Each of these features needs to be tested independently, and the README or appropriate doc updated as to how to enable/disable and use each feature.

[breezykermo]

Additional features:

• CATEGORIES_AS_TAGS, which creates special tags for each category that compose with other tags via intersection. This can only be enabled if USE_TAGS is enabled as well.
• NARRATIVE_STEP_STYLES, which allows styling to be specified per step in each narrative (rather than steps being uniformly styled within each narrative). Requires an additional narrative___stepStyles array in each event that indicates the name of the style that should be applied, the indices of which correspond with the narratives array in the same event. Styles are specified in the Redux store.