Verifying understanding of RedCarpet options, pt 4

Documented current behaviour of/defaults for
`:disable_indented_code_blocks` option.

RSpec: 474 examples, 0 failures
RuboCop: 127 files inspected, no offenses detected
Verifying understanding of RedCarpet options, pt 5

Documented current behaviour of/defaults for
`:strikethrough` option.

RSpec: 477 examples, 0 failures
RuboCop: 127 files inspected, no offenses detected
Verifying understanding of RedCarpet options, pt 6

The `:lax_spacing` option is documented but has no apparent effect; we
get the code block either way. Pffft.

Now using the master-branch version of `redcarpet` because there have
been so many commits since the last formal release. Changing this had
no effect on `:lax_spacing`.

RSpec: 479 examples, 0 failures
RuboCop: 127 files inspected, no offenses detected