add floating point output to math command

This makes it easy for the user to request floating point output with the
desired number of digits after the decimal point (not to be confused with
significant digits).

Note that this is just a thin wrapper so someone can say `math -s3 10 / 3`
rather than `math "scale=3; 10 /3"`.

Resolves #1643

tests/math.out

@@ -0,0 +1,8 @@
+1
+1
+1
+1.666
+4
+4
+2
+.499999