Support custom unit display in order to handle "$10,000"

[simonw]

I tried to get Datasette to display $10,000 using the new units support but we currently only display units as a suffix:

datasette/datasette/app.py

Lines 563 to 572 in 10a34f9

	if column in table_metadata.get('units', {}) and value != '':
	# Interpret units using pint
	value = value * ureg(table_metadata['units'][column])
	# Pint uses floating point which sometimes introduces errors in the compact
	# representation, which we have to round off to avoid ugliness. In the vast
	# majority of cases this rounding will be inconsequential. I hope.
	value = round(value.to_compact(), 6)
	display_value = jinja2.Markup('{:~P}'.format(value).replace(' ', ' '))
	else:
	display_value = str(value)

It would be neat if there was a mechanism for specifying a custom unit display - maybe something like this:

{
    "custom_units": {
        "us_dollar": {
            "unit": "us_dollar = [] = $",
            "format": "${:,}"
        }
    }
}