zencan-cli write command cannot accept negative integers

Proud of this one. The suggested workaround works...but hopefully can be fixed to not require that

```
can0>write 10 0x2010 3 i16 -46 
error: unexpected argument '-4' found

  tip: to pass '-4' as a value, use '-- -4'

Usage: write <NODE_ID> <INDEX> <SUB> <DATA_TYPE> <VALUE>
```