reader: support more formats

[adamdecaf]

What were you trying to do?
There are several "microformats" of X9 files and vendors. These vary with prefix words, character encoding, image formats, and others. Ideally our ICL library would handle parsing all these formats. The reader could be restricted to a specific, but always report which format was used to parse the file.

What did you expect to see?
On Reader we should have a Format enum which by default is Discover and could hold other values (such as X9100, X9137, etc..).

Adding (r *Reader) SetFormat(value Format) and a Format member on File that the reader populates.