Allow customizing quote char in xml header

[cmyr]

• add a quote_char method to WriteOptions
• When serializing the .glifs, (which we serialize manually) write the header to the stream ourselves, using the specified character
• When serializing plists, we don't control writing the header, so we'll have to rewrite. There are options here. All plist writing is now handled through a method in write.rs.
  • One option is to write to disk, and then use the unix/windows specific methods to go and overwrite the header in place, before closing the file handle: see std::os::unix::fs::FileExt and std::os::windows::fs::FileExt; you'll need slightly different impls on either platform.
  • the other option would be to have a custom Write type, that would wrap or replace the BufWriter; this would intercept the header and replace it, if needed, otherwise forwarding the bytes to the inner writer.

Thinking about this, I think the first option is totally reasonable. The second option is probably marginally more 'efficient', but I don't think it matters; the main thing we want to avoid is closing and reopening all the file handles.

Originally posted by @cmyr in #148 (comment)

[chrissimpkins]

Started

[chrissimpkins]

#157

Let me know what you think of the builder enum definition approach.

e.g.,

let opt = WriteOptions::default().whitespace("  ").quote_char(QuoteStyle::Single);

Currently supports glif XML writes. I'll have a look into plist support as you suggested above.

[chrissimpkins]

Optional XML declaration single quote formatting is supported in plist files as of 2a962cd