(Long-term) Make minor extensions to SMIRFF format

[davidlmobley]

In the issue discussing the SMIRFF format (https://github.com/open-forcefield-group/open-forcefield-data/issues/9 ) we laid out a number of longer-term additions we wanted to make to the format. This is to remind us to make these changes eventually:

• We will make a modification of the file format to specify a prior for a parameter search (https://github.com/open-forcefield-group/open-forcefield-data/issues/9#issuecomment-219327074)
• We will extend the format to allow macros such as "smallhals" in SMIRKS strings (https://github.com/open-forcefield-group/open-forcefield-data/issues/9#issuecomment-222592960 )
• Files will include parameter id and parent_id which track their origin

Ultimately we can accommodate fractional bond order as well: https://github.com/open-forcefield-group/open-forcefield-data/issues/9#issuecomment-224364826

[davidlmobley]

id and parent_id were added in a recent PR. The other changes will still be needed.