Support baking new RST files

[Crauzer]

Seems like the RST crate currently supports only parsing.
I'll try looking at the format and make a builder API for the crate so that we can create the files from scratch or edit existing ones.

[benoitryder]

The current Rst struct is designed around parsing, with lazy interation/parsing. It would be better for the "write API" to be separated.

An approach could be to let the user provide a map of strings (indexed by strings or hashes; both could be supported) and provide a write function (or writer struct); something like this:

fn write_rst(writer: Write, header: RstHeader, entries: HashMap) -> io::Result<()>;

This function would be in charge of finding duplicates, sorting entries (if needed), etc.
The RstHeader would provide the version, "font config", etc. It could be used by the Rst struct too, and thus can be easily moved from parsing result to edit an existing file.