Add support for placeholders

[kevinlang]

This will likely require support from exqlite for named paramters: https://www.sqlite.org/c3ref/bind_blob.html

Exqlite will need to handle taking in a parameter name and finding the correct parameter index via sqlite3_bind_parameter_index() when it does its binding, if I am understanding correctly.

[warmwaffles]

I don't understand what placeholders are supposed to be. Other than the named ?1 within the prepared statements. That said, exposing those bindings cleanly will be pretty easy.