Improvements to generated PDF document

[stmio]

This PR fixes a few errors with the automatically generated PDF documents, as discussed in #32:

• Moves the "Aseprite Help" (overview.md) chapter to the top of the document.
• The main drawback of the PDF in its current state is the lack of GIF animations. This adds a pandoc Lua Filter, which programmatically adds a hyperlink to the online copy of the GIF, so they can be viewed in the user's browser when the GIF is clicked.
• Add a notice at the top of the document explaining the ability to view GIF animations