Although `yes` and `no` are recognized as booleans by the pyyaml parser,
the correct keywords are `true` and `false` (as highlighted by the newly
added `truthy` rule).
This commit replaces the use of `yes`/`no` by `true`/`false` and
advertise it in the docs, but also makes sure this change is
backward-compatible (so that `yes` and `no` still work).
This change make the `comments` rule accept comments that start with
multiple pound signs, e.g.:
##############################
## This is some documentation
Closes: #12
YAML content like the following one produced an error, because the
ScalarToken associated whose value is "this is plain text" ends at the
beginning of the 5th line (the one with the comment):
---
string: >
this is plain text
# comment
