Add option for lineJoin shape

This commit adds an option for the shape of the corners of the
selection rectangle. By default the shape is set to "round" (the
previous setting for lineJoin). The other options are "bevel" and
"miter".
Add option to always show selection rectangle

Previously, if the selected area was very small, the selection
rectangle would not be displayed. This commit adds an "alwaysShow"
option so that, when true, the selection rectangle will always be
displayed. When the selected area is very small, the selection
rectangle will become a line.
Make minSize customizable

Previously, the minimum size a selection could be was set at five
pixels. This commit adds the ability to customize this value.
Merge pull request #963 from rlinehan/selection_plugin_options

Added selection plugin options.