Add ability to toggle the commits graph via env variable

[zdoc01]

Hey @notwaldorf! Thanks for taking the time to build and open source this awesome tool! I love it.

I tend to keep my terminal split into multiple panes, and I noticed the commits graph seems to have rendering issues when in smaller viewports, so I thought it would be helpful if I could easily hide it via environment variables (sort of aligns with #109). I do still like having the total number of commits easily visible for a given day and week, so I added them to the Today and Week box labels when the commits graph is hidden. Here's a quick look:

Before

After

The PR itself mostly just consists of introducing the new config value and adjusting the size of the boxes accordingly, as well as adding the total number of commits to the box labels.

Let me know what you think!

[mojoaxel]

@zdoc01 Thanks for your contribution!