Parse all directives, and merge/replace them

[xbc5]

This relates to #6.

The default is merge -- always.

Front-matter takes precedence, then URI specific directives, then the default directives.

First, merge/replace the production environment, then merge/replace with development environment. These respect the env mode and individual frontmatter/URI modes.

csp:
  mode:
    env: merge
  prod:
    directives:
      foo/index.html
      mode: replace

This is frontmatter, it replaces the layer below it.

---
csp:
  mode: replace
---

Examples:

If front-matter specifies replace, then that is the final form. If that's then merged with development environment, then it's a mix of the two.

If front-matter is merge and URI is replace, then the final form is front matter + URI (replacing default).

[xbc5]

Replace is not fully implemented. Re-opening.