Add set_limit and helpers for enabling and disabling

[vectorcatch]

I saw the other pull request regarding the limits, however, I did not have the same results that were mentioned in the other pull request.

For me if I set the charger to 0 AMPS it stays at 0 AMPS. I think perhaps there are some nuances to set limit for cases between 0 and the max limit that I haven't fully explored. However, for the case I am most interested (Home Assistant) it looks like we can safely enable/disable at the charger using 0 and the Max charging rate.

[nlarbale]

Any plans to merge this? I'd love to have this functionality so I can create integrations to turn off charging if off grid for example

[jesserockz]

Thanks