Add list_pages/3 function

[goncalotomas]

Previously, Collections were the only way of listing pages, which is required for building index pages. Looking after the source code after a while made me realize that I really value simplicity, and Collections were too much logic in a single mechanism, especially for simple use cases.

The question raised in #12 is pertinent: it shouldn't be that complicated to list pages, so these changes add a function that allows listing pages with a simple filter and sort option.

How it works

Griffin works in two stages: parsing, where files are read and split into a {frontmatter, content} pair, and rendering, which is when files are written to the output directory. This logic has been extended a bit to also store the path the file comes from relative to the root directory, so that it is searchable.

The frontmatter/metadata is really all that's needed for listing pages, so we're now storing the parsed file metadata in ETS.

In the rendering stage, we now inject a @list_pages assign as a function wrapping GriffinSSG.list_pages/3 which enables listing pages from layouts.