Add set_if(), prepend_if(), and append_if() support

Use set_if(), to conditionally set a specific value.  This will be
supported for all variable types.  Example:
   FOOBAR = "foo"
   BAR = "f"
   FOOBAR.set_if(BAR=='f', 'bar')
which should yield FOOBAR='bar'.

For sequence types, prepend_if and append_if will conditionally prepend
and append values.

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Change str() function behavior on unset Variables

Instead of returning None, return empty string ('').

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Add Bool, Int and Float variable support

Signed-off-by: Esben Haabendal <esben@haabendal.dk>