Add a psplot command to plot and compare stats from multiple runs of psrecord

This allow to compare multiple runs of psrecord on one plot. I have added a new command since it is easier than playing with argparse's sub-parsers.
It takes a list of log files from psrecord, with options to plot only mem or cpu. I don't know what is the best for colors and linestyles to distinguish easily all the lines (suggestions are welcome ;)). Currently it uses mpl's color cycle, so mem and cpu for a file will have the same color (and linestyle).
I didn't modify the plot from `main.py`, it could probably use the same code, but I'm waiting for your feedback before going further ;-).

```
usage: psplot [-h] [-m] [-c] [-o OUTPUT] [files [files ...]]

Plot stats from psrecord

positional arguments:
  files                 List of files to plot

optional arguments:
  -h, --help            show this help message and exit
  -m, --mem             Show memory values
  -c, --cpu             Show cpu values
  -o OUTPUT, --output OUTPUT
                        Save the plot to a file
```