Adds parameter substitution for start_date and end_date

This lets you specify {{{start_date}}} and/or {{{end_date}}} in sql queries and they get automatically substituted to create a default date range of two weeks ending yesterday. Controls appear above edit chart when these are used. I used mustache because it's convenient, but I'd be open to something else. It's a little annoying to have to use triple braces to avoid html substitutions.
Hmm, I find references to the `parameters` and `parameter_defaults` methods in the `parameters.rb` file confusing, because I keep on expecting them to be local variables instead. Should we stick those things into a Module maybe?