Added option to start with no handle by setting default_position to :none
Moved default_position logic to the dsl.

Closes #106

LGTM given by: @esposito

Squashed commit of the following:

commit dbf59c97389a37e3f58db2ac94b9e17f1e5f0b57
    moved slider default_position middle to dsl
commit 195699a6fd2a3eb95346c33d2c9b9e6282cbb5b8
    added option to start with no handle by setting default_position below the min value