feat: let the user set the displayed precision of the time

[ClementWalter]

Why

Currently, the displayed precision of the time is hard coded to 4 in the backend and 3 in the frontend

• https://github.com/jiffyclub/snakeviz/blob/master/snakeviz/stats.py#L28
• https://github.com/jiffyclub/snakeviz/blob/master/snakeviz/static/drawsvg.js#L176
• https://github.com/jiffyclub/snakeviz/blob/master/snakeviz/static/drawsvg.js#L295

This results in the table stat giving 4 digits precision, the sv-info panel 3 digits precision, and no room for the user to recover the actual numbers.

What

Let the user define the displayed precision.

How

1. use a single precision variable
2. truncate in js using this number for both the chart data and the panel