Better string interpolation

The standard String Interpolation (e.g. `s"some $string interpolation"`) can be mixed with Anorm one.

```scala
val cmd = "SELECT"
val table = "Test"

SQL"""#$cmd * FROM #$table WHERE id = ${"id1"} AND code IN (${Seq(2, 5)})"""

// prepare the SQL statement, with 1 string and 2 integer parameters:
// SELECT * FROM Test WHERE id = ? AND code IN (?, ?)
```
I think this needs to be explained a little clearer, it took me a while to work out what it was actually saying.  Perhaps something like this:

> SQL string interpolation also supports direct substitution of interpolated variables, in addition to binding interpolated variables.  This can be done by prefixing the `$` with `#`, for example, `#$cmd`.  Using this syntax will result in variables being directly substituted into the SQL query, rather than being parameterised and bound.  For example, the following string:

```scala
val cmd = "SELECT"
val table = "Test"
val id = 10

SQL"""#$cmd * FROM #$table WHERE id = $id AND code IN (${Seq(2, 5)})"""
````

> is equivalent to:

```scala
SQL("SELECT * FROM Test WHERE id = {id} AND code IN ({seq})")
  .on("id" -> id, "seq" -> Seq(2, 5))
```