Convert `Config` to only use for `build()`, not use for `view()`

[lwjohnst86]

Sorry, just had another thought:
Now that Section is not part of Config, view doesn’t actually use Config anymore. build_sections takes a Config arg, but it could just as easily take style and template_dir. It doesn’t even need output_dir.

If we refactored the design for view and build_sections like so, then Config could be a bit simpler, because it wouldn’t need to take into account the needs of view. For example, the Style enum input to Config wouldn’t need to include view styles.

Originally posted by @martonvago in #102 (review)