Update to doctemplates-0.9

0.9
* Add BoolVal constructor to Val. This gives a smoother interface with
  JSON and YAML. [API change]
* Remove overlapping instances by generalizing ToContext String String
  and FromContext String String to TemplateTarget [a] => ToContext [a]
  [a] and TemplateTarget [a] => FromContext [a] [a]. Remove the
  instance ToContext String (Doc String). Remove redundant
  constraints. (#9, favonia) [API change]

0.8.3
* Properly handle nested loops (#15). Previously "it" was always used
  for the variable in a loop, and in a nested loop there was no way to
  distinguish the value of the inner iteration from the value of the
  outer one. Now we assign the iterated value to both "it" and to the
  original variable name (e.g. "foo.bar"). This probably has a small
  negative performance impact. Note that this change also affects the
  output of the template parser: original variable names are now
  retained instead of being replaced by "it".
* Remove duplicate IsString constraint (#14, Mario Lang).
* Update haddocks from README (#10).
* Minor code clean-ups (#7, favonia).
* Add hsyaml >= 0.2 constraint (#6).

0.8.2
* Add filters: first, rest, last, allbutlast.
* New constructors for Filter: FirstItem, LastItem, Rest, AllButLast
  [API change].


(pho)