Generalized code for Load to use io.Reader

Here is proposed implementation of generalized Load() using io.Reader.
Also three wrappers are available: LoadFile(), LoadString() and LoadBytes().
If you choose default Load() to use os.File, and have separate LoadReader() call to use Reader - it's ok with me, feel free to decide. But this approach is probably the most idiomatic.

Also, I've changed package name in dotsql_test.go - to be able examine dotsql.queries length. Tell me if it's ok with you.