add generalized extractors for Mastodon instances (#144)

Extractors for Mastodon instances can now be dynamically generated,
based on the instance names in the 'extractor.mastodon.*' config path.

Example:
{
    "extractor": {
        "mastodon": {
            "pawoo.net": { ... },
            "mastodon.xyz": { ... },
            "tabletop.social": { ... },
            ...
        }
    }
}

Each entry requires an 'access-token' value, which can be generated with
'gallery-dl oauth:mastodon:<instance URL>'.
An 'access-token' (as well as a 'client-id' and 'client-secret') for
pawoo.net is always available, but can be overwritten as necessary.

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Changelog
 
+## Unreleased
+
 ## 1.6.3 - 2019-01-18
 - Added `metadata` post-processor to write image metadata to an external file ([#135](https://github.com/mikf/gallery-dl/issues/135))
 - Added option to reverse chapter order of manga extractors ([#149](https://github.com/mikf/gallery-dl/issues/149))