Add getPosition() function to Reader

[bingkh]

Hi,

Description
As a user of ion-js, I would love to be able to get position information from ionReader, so that when using ionReader to read a ion file, I can know which position I'm currently at.

Application scenario
Our team uses ion to define a format, e.g. paragraph P uses certain format style S, and the definition of style S is out side of the paragraph itself. see blow:

style::{
  name:S,
  font:Arial,
  .....
}
....
....
paragraph::{
  name:A,
  style:S,
  .....
}

I'm working on a vs code extension: ion-style-peek, so that in the above ion file, when users are viewing paragraph A and call go to definition on S, the editor will jump to style S. Similar implementation can be found: https://github.com/pranaygp/vscode-css-peek

To implement the above, we need to get the position when parsing style container in the ion, hence the requesting issue.

Pull request:
#655

Thanks!

Ethan

[zslayton]

Added in #657.