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)
fix for passthrough, quote & anchor subs; register reference method

- multi-line passthrough, quote escaping, quoted xrefs and anchors:

- change the passthrough macro regexp to support multiple lines
- update unconstrained quoted text regexps to support escaping
- handle quoted anchors and xrefs
- tests for escaping all forms of quoted text

- add register method for ids and make references extensible:

- make references a hash of key=>collection pairs
- store id references in the :ids key of references
- catalog images and links
- remove deprecated anchor from abstract block
- use quoted strings for interpolation as it has better performance
- add to_s methods to make it easier to inspect the document structure

- add tests for blocks adjacent to paragraphs
Merge pull request #65 from mojavelinux/compliance-fixes

Various compliance fixes
parse document directly instead of chunking into section lines

- remove the section reader and instead parse the sections off main reader
- add parse method that handles parsing logic previously in document
- parse downwards from sections rather than through the next_block method
- add mode to next_block method where it only looks for blocks
- make section title parsing more efficient
- rearrange some of the methods in Lexer
- rename heading_* symbols to section_*
- add tests for block level
- TomDoc
- some source code tidying
add test for block title carrying over from section to first block
fix source of infinite loop
support square & round brackets in front of link, round brackets around a link
set asciidoctor attribute
prevent empty dd element for item w/ multiple terms
make sure nested labeled lists are found when indented

- nested labeled lists may occur in indented paragraph text
- but not if literal paragraph is attached by list continuation
prevent literal & indented content from being too greedy

- fix literal content in a list item from consuming following list item
- fix indented content in labeled list from consuming following content
- more list tests
- add convenience methods for checking if blocks, text or title exists
change keep_if to delete_if for ruby 1.8.7
additional list test for title on following block
fix bad reference to safe-paths, don't clobber value