Add way to read only the header of an OSM file.
Add descriptions to FileType and Encoding.
Rename osmium::OSMFile to osmium::io::File
Rename osmium::io::Meta to osmium::io::Header

This is no general "meta data", but more specifically the data from the header
of OSM files. This also differentiates it from other uses of the word "meta"
in Osmium.