In order to keep compatibility to before allowing multiple marks on a window,
we introduce a flag "--add" that must be set to put more than one mark on a
window. The default, which is also available as "--replace", keeps the old
behavior of overwriting a mark when setting a new one.
fixes#2014
This patch allows multiple marks to be set on a single window. The restriction that a mark may
only be on one window at a time is still upheld as this is necessary for commands like
"move window to mark" to make sense.
relates to #2014
Ian Fan