Allow caller to specify path/line when rendering with Proc or String.

When rendering a template from a supplied `Proc` or `String`, Sinatra generates the path/line information for Tilt via `settings.caller_locations`. This is a reasonable default, but the caller may wish to override the default (for example, if there happens to be a filesystem path associated with the String). This patch allows the caller to pass those values in the `options` parameter.