Add WikiPageRefile functionality

[lervag]

This PR adds a WikiPageRefile command (with mapping) for moving a section from one page to another. This was asked for in #58.

It is not quite ready. The idea is to have the function wiki#page#refile respect the following options:

• target_file: Move section to target file (use current file if this is not provided)
• target_lnum: Move section to below specified line number in the target file
• target_anchor: Move section into the section specified by anchor

Tasks:

• Allow empty target_file for moving sections within the current file.
• Allow to specify target_anchor for moving one section into another one.
  • This should automatically adjust the header levels of the refiled section with subsections as appropriate
• All of the specified features should have good tests
  • Test that bad input gives proper warnings
  • Test that we can move a section within the current document and that it will update links accordingly
  • Test that we can move a section to a specified target section (with target_anchor)