[add] num command

[Mc-Zen]

• proper number parsing
• digit grouping options
• customizable decimal-marker
• uncertainty: automatically equalize precision between value and uncertainty
• tight number formatting
• implicit + in front of positive numbers -

Also fixes:

• parentheses around number in combination with num

Also adds:

• allow-breaking for qty

Actually I noticed, the _state dictionary need not contain any default keys if the internal _unit and num-impl functions have default values for these keys (see the definition of num-impl). This way, the _state contains less key-value pairs in most cases and the overhead of updating the '..options' in num, qty and unit is reduced. This could also be applied to '_unit' in a similar fashion.

[fenjalien]

It looks good! Thank you :)