Support parsing XMLs with integers in bases different than 10

[KenjiTakahashi]

Some folks over at NetBSD like their XMLs with hex encoded integers. Since strconv parsers can derive base on their own, I think it will be best to let them do it.

[dmitshur]

@DHowett any review comments on this PR? Should it be accepted or rejected? What would it take to make progress here?

I'm not very familiar with go-plist and its details, but this change seems reasonable (and backwards compatible) to me.

[DHowett]

Interesting. This change absolutely makes sense.

On OS X, hex int literals in plists work fine, but octal ones are ignored (treated as decimal). I'm willing to break with compatibility to support octal literals, if there's a compelling reason.

If not, we should retool this so that it only switches to base 16 with the 0x prefix.

[KenjiTakahashi]

I don't have any reason for octal numbers support, other than it being easier to implement. I need only hex.
If you wish to stick to what OS X does, I'd adjust this change to only threat hex numbers specially.

[DHowett]

That would be great! Let's stick with what OS X does.

[KenjiTakahashi]

Sorry for such a long delay, real life has caught up with me :-). Updated now.

[DHowett]

This looks great. Would you mind throwing in at least one positive and one negative decode test?

[KenjiTakahashi]

@DHowett Updated. Let me know if there's anything else.

[DHowett]

Thanks a lot for contributing, and waiting, and putting up with my change requests!