handle the {index} parameter like a native MessageFormat argument.

[melchisedek]

For example that allows to specified leading zero by using {index,number,0000}

[kcooney]

Please add some tests for this.

[avandeursen]

Looks like an interesting addition.

This also requires adjusting the documentation.

I would suggest adding an extra paragraph (after line 75 in the changed file) giving a number of insightful and compelling examples on how to make good use of this new feature.

[adrianosimoes]

Should the conflicts be fixed, or maybe it's too late and we should just close this pull request?