tableAppend - Easily append selected variables to table

It usually happens that you want to add rows to a table but you only have part of the original variables. There is currently no way to do this without having to create a table row with the complete list of variables.
tableAppend will concatenate the variables that are present in the original table, and will pad with NaN or 'NA' the missing variables.

Example:

tbl1:

var 1var2 var3
---- ---- ----
'a' 45 6
'b' 14 3
'c' 56 5

tbl2:

var3
----
8
2

tbl3 = tableAppend(tbl1, tbl2);

tbl3:

var1 var2 var3
---- ---- ----
'a' 45 6
'b' 14 3
'c' 56 5
'NA' NaN 8
'NA' NaN 2