Add literal support and concatenation to SPECTQL

[lievenjanssen]

Implement ability to concatenate column values and strings.

e.g.

/person{name, 'http://icons.com/' + gender + '.png'}

would generate an icon url for a gender icon which could be used on maps.

(btw would love to have a full implementation of HTSQL standard in SPECTQL, will log a seperate issue for that for a next release)