add xmlReadFile and parse_file with options

[tkelman]

This is useful for setting options flags, ref http://www.xmlsoft.org/html/libxml-parser.html#xmlReadFile and http://www.xmlsoft.org/html/libxml-parser.html#xmlParserOption

TODO: tests and docs done

[timholy]

LGTM

[stevengj]

second argument should be Cstring too, since it is presumably a NUL-terminated string (you aren't passing a length). And should it be encoding::AbstractString?

[stevengj]

Oh, I see, you want to be able to pass C_NULL for encoding. That seems pretty un-Julian to expose. Maybe just have a separate parse_file(filename, options) method? (Or make encoding the third option, since in Julia it is more typical to put optional arguments last.)

[tkelman]

If I wanted this to be much prettier I'd probably do a keyword-argument version with all the option enums spelled out. I could maybe have called this read_file to make it more clearly a direct mapping of the C API, but doesn't seem worth the trouble to have a different argument order.

[stevengj]

The problem with not using Cstring is that you get silent truncation of filename strings with embedded NUL chars.

[tkelman]

Just encoding strings with embedded nul's. I'd rather have this API be closer to the extensively documented upstream C API, even if that means a few methods are a little unsafe. There are bigger issues to worry about here, like memory management being kind of a mess without reference counts.