implement passthrough block

- add passthrough block
- rename regex for lit_blk to literal for dynamic assignment of context
- add resolve_subs() option for resolving subs for user-supplied list
- tests for passthrough block and passthrough substitutions
fixes issue #63, options to disable includes

- leave escaped include::macro unprocessed (but strip backslash)
- disable includes if include-depth attribute is 0
- include-depth attribute cannot be assigned in the document (it's "immutable")
section numbering

- keep track of the index of each section as it's added to its parent
- create a method for generating a section number
- add section number support to backend templates
- add convenience method to retrieve child sections from a parent block
- tests!
Merge pull request #66 from mojavelinux/no-include

options to disable includes
Merge pull request #64 from mojavelinux/passblock

Implement passthrough block
Merge pull request #68 from mojavelinux/section-numbering

Section numbering
Make tilt a development dependency, not a runtime one (fixes #54)
merge upstream changes