Improve Page Rename

[redimp]

As discussed in #210, renaming a page could make use of some features:

• a) the links to attachments on the page itself (and other pages, using the same attachments)
• b) the links to the page form other pages
• c) corner case: When renaming attachments, the links shoudl be updated, see Renaming attachments should updated links in pages. #65

These should be optional when renaming pages, so that you have to check a box, when you want links to be auto updated. There are cases, where users rename pages on a regular basis and don't want the links to be updated.

[skirmess]

I took a stab at this as an example plugin: #508.

It updates WikiLinks pointing to a page when that page is renamed, via the page_renamed hook. Anchors ([[Page#section]]) and explicit [[title|link]] titles are preserved, and matching goes through get_filename() so it's case-/slash-/.md-insensitive.

Scope is intentionally limited to WikiLinks for now (plain Markdown links and attachments/#65 are not covered). Since a per-rename checkbox isn't reachable from the current plugin API, it uses an UPDATE_LINKS_ON_RENAME config flag (default on) so it can be disabled without uninstalling — e.g. when baked into a Docker image.