Fix issue #55.

This issue is related to how strings for select backends are not deeply
copied, but rather only have their pointers copied. This results in a
situation where the client may delete the strings they passed to the
context config assuming miniaudio makes a copy of it when in fact it
hasn't.

* To read bytes, use ma_decoder_read_bytes().
* To seek bytes, use ma_decoder_seek_bytes().
* The read pointer is now tracked in the ma_decoder object. You can use
  this to move back to a prior position.