Human readable output for data read command

Default output from the _data read_ command is now human readable, with a `--raw` option to return the actual values (as rendered by their Show instances). The output formats dates in ISO 8601 and simple payloads as either integers or floats, according to their magnitude.

AfC
It's not immediately obvious to me what gets parsed as an int or float/what the significance of 2^51 is (one less than the number of bits in the significand?). Explanatory comment would be handy here.