Windows, and to a lesser extent OS X, don't conform to XDG and have
their own preferred locations for caches.
On Windows, use %LOCALAPPDATA%/restic (i.e., ~/AppData/Local/restic). I
can't find authoritative documentation from Microsoft recommending
specifically which of %APPDATA%, %LOCALAPPDATA%, and %TEMP% should be
used for caches, but %LOCALAPPDATA% is where browsers store their
caches, so it seems like a good fit.
On OS X, use ~/Library/Caches/restic, which is recommended by the Apple
documentation. They do suggest using the application "bundle identifier"
as the base folder name, but restic doesn't have one, so I just used
"restic".
This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.
