Add a 'o' modifier to the ~x sigil allowing a path to not exist

[benwilson512]

Motivation:

I'm the author if ExAws, a set of elixir clients for interacting with AWS, and have been using sweet_xml to parse the resulting xml. This has been an extremely helpful library for handling xml results returned by AWS.

However, as it turns out, AWS will often leave out portions of xml depending on permissions, request parameters, and so on. Presently this will return the following error:

     ** (Protocol.UndefinedError) protocol Enumerable not implemented for nil
     stacktrace:
       (elixir) lib/enum.ex:1: Enumerable.impl_for!/1
       (elixir) lib/enum.ex:112: Enumerable.reduce/3
       (elixir) lib/stream.ex:695: Stream.do_enum_transform/8
       (elixir) lib/stream.ex:647: Stream.do_transform/7
       (sweet_xml) lib/sweet_xml.ex:513: anonymous fn/4 in SweetXml.continuation_opts/2
       xmerl_scan.erl:563: :xmerl_scan.scan_document/2
       xmerl_scan.erl:286: :xmerl_scan.string/2
       (sweet_xml) lib/sweet_xml.ex:189: SweetXml.parse/2

What this PR does is add an 'o' modifier ~x"//some/bad/path"o such that it will return nil if the path does not exist instead of exploding.

[benwilson512]

Note that this clause here ensures that the changes to xmap are not breaking. All current uses will pass without trouble.

[benwilson512]

Are there any outstanding questions or concerns?

[viniciussbs]

Hi, @benwilson512. A modifier for optional paths is a nice feature. Could you updated the README, too, describing this feature? I'm not a maintainer, I'm just trying to help.

[benwilson512]

Done.

[benwilson512]

Hey, any possibility for movement on this?

[viniciussbs]

I'm not a maintainer, but 👍 What do you thing, @awetzel ?

[awetzel]

Yes it is perfect sorry for the late answer, I had already reviewed your commit and thought that it was already merged... Thank you very much for the contribution, I will publish to hex next week maybe with other improvements.