Introduce the stash view

This patch adds a new "stash" view, bound to the 'y' key.
This view displays the list of stashes. Pressing Enter on one of them
displays the corresponding change in a split view.

A new %(stash) variable is added for bindings and commands.

Running 'tig stash' opens directly the stash view.

A built-in command to pop the current stash is bound to 'P' in this
view. Other bindings may be added in the ~/.tigrc file, for instance:

    bind stash D !?git stash drop %(stash)
    bind stash A !?git stash apply %(stash)
    bind status S !?git stash save --keep-index %(prompt)

References: #169, #159

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>