move output definitions to _data/format.yml

currently any change to _config.yml triggers a full rebuild, since the pandoc generator considers it a source file.

it would be easier to make changes to an individual format without having to regenerate the whole site. for instance:

• _data/pandoc.yml would contain the configuration (it can remain in _config.yml)
• _data/pdf.yml would contain pdf specific flags
• _data/epub.yml idem

that way the generator can check which source file has been modified to know which formats need to be rebuilt.

_config.yml or _pandoc.yml could still contain the outputs (or rename it to formats) array to know which formats are enabled.

for retrocompatibility, the config loader could just build the outputs hash from the data files. the source checker needs to find the files though