Can be used to specify what file the url is added to. This can be used to
override the default filename that is used when adding an url, which is
based on the url. Or, when the file already exists, the url is recorded as
another location of the file.
Made --from and --to command-specific options.
Added generic storage for values of command-specific options,
which allows removing some of the special case fields in AnnexState.
(Also added generic storage for command-specific flags, although there are
not yet any.)
Note that this storage uses a Map, so repeatedly looking up the same value
is slightly more expensive than looking up an AnnexState field. But, the
value can be looked up once in the seek stage, transformed as necessary,
and passed in a closure to the start stage, and this avoids that overhead.
Still, I'm hesitant to use this for things like force or fast flags.
It's probably best to reserve it for flags that are only used by a few
commands, or options like --from and --to that it's important only be
allowed to be used with commands that implement them, to avoid user
confusion.
