Add user-specific conversions

[liz-baker]

From discord:

Nadine: yknow it would be cool if it defaulted into mg/dL for me unless otherwisr specified

Might be interesting to add a setunits command for something like this:

<dosman711> .setunits mg/dl
<diabot> dosman711: default units set to mg/dL
<dosman711> 10
<diabot> 10 mg/dL is 0.6 mmol/L