This branch makes the following changes:

1. Adds new configuration options document_roots and websocket_roots. These serve the same function as the existing document_root and websocket_root, except they allow the user to specify multiple directory-paths, colon-separated, to serve content from. (Under Windows the directory-paths should be semicolon-separated instead, since Windows uses colons in its drive-letter designators)
2. Deprecated the fallback_document_root and fallback_websocket_root options, since they provide the same functionality as the new options except in a more limited fashion.
3. Added internal/private function mg_setup_document_roots_vector() which allocates and returns an argv-style NULL-terminated array of path strings based on the specified options. The paths-array will contain the path specified by document_root first (if any), followed by the string specified by fallback_document_root (if any), followed by each of the strings specified by document_roots, followed by a NULL entry. This array is stored as member-variable document_roots in the mg_domain_context class.
4. Modified the file-path-lookup code to iterate over the document_roots array instead of consulting the config options directly.
5. Made analogous changes for websocket_roots.
6. Added internal/private function mg_free_dom() for code that wants to deallocate a mg_domain_context object. This function will take care to deallocate the document_roots and websocket_roots allocations owned by that object as well.
7. Updated the documentation to include the new keywords.