custom axis formatters on plot

[carlverge]

This adds options on Plots for custom formatting on X and Y axis labels. Example use-cases include timestamp formatters on the X axis, and percentage / bps formatters on the Y axis.

[avivhaber]

In the current code, it looks like 0 is hardcoded to be the first value on the x-axis. Code here. We should call XAxisFmter(0) in that spot to replace it

[dcorbe]

Hello!

Thank you for contributing to termui. I've merged this PR into a downstream fork at https://github.com/dcorbe/termui-dpc

This was done in response to the following issue: #304 (comment)

My intention is not to create a permanent fork for termui. I'm only trying to facilitate maintenance while the original author(s) are away. My hope is that my fork is eventually merged back into this repo along with your commits.

I would like to invite you to collaborate further. I would appreciate any help curating fixes and generally getting this project going in the right direction. Thanks again for your contribution!