Custom Label Format

[mstainer14]

Added custom label formatter so that the user can customise how the label appears on the gauge.
#280 Linked to the first part of this issue.

Example usage:

 const LinearGauge(
  pointer: Pointer(
    labelFormatter: (double? value) => '$value%',
  ),
 ),

I have not updated the pubspec.yaml versioning in this PR in case you have some other changes you want to bundle in with the next release. If you wish for me to do that I can. Additionally, any questions about the PR or any futher changes you wish me to make, please let me know.

[mstainer14]

@Afroz-Shaikh

[Afroz-Shaikh]

Hey @mstainer14, thanks for submitting the PR! I'll review it when I get a moment. It looks quite similar to what we've been implementing in an another branch, but I'll go through it thoroughly and get back to you with feedback.