#18 Spreadsheet: Add increment fill

=1 INCR
=3 INCR 2
=0 INCR 0.1

=1 INCR     Fill down
=1 INCRV    Fill down
=1 INCRH    Fill right

Should be able to autofill a column with incrementing numbers, akin to how dragging a value cell in a spreadsheet app will usually fill subsequent cells with increasing values. Proposed syntax: ``` =1 INCR =3 INCR 2 =0 INCR 0.1 ``` Maybe also do: ``` =1 INCR Fill down =1 INCRV Fill down =1 INCRH Fill right ```