PER-85: Mustache partials

This changeset enables us to use partials in Mustache templates in our codebase.
Partials are automatically scanned and prefetched before invoking Mustache renderer.
Partial names are resolved relative to currently processed file, so you can specify a relative path to partial (don't specify the .mustache extension as it's always appended), eg.:
- "item", "./item" - item.mustache in the same directory,
- "../item" - item.mustache in parent directory,
- "subdir/item", "./subdir/item" - item.mustache in subdirectory called "subdir".

@Wyvek @sebastian-wikia @owend @federico-lox You're welcome to review this code.