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.