New supported option called 'pinned' for the Plugin command. When set to
1, the plugin is added to rtp but no install/upgrade operation is
performed.

It not only allows vundle to do rtp management of plugins on VCS other
than git, it also allows leaving plugins that had previously been
managed by vundle in the current state, with no further updates.

Fixes #24, #397

- closes many issues )

Bundle configuration options that are inferred from the bundle specs
overwrite user provided ones. This used to prevent users from changing
the name of directory where the cloned bundle should go, for instance.

Keep the user provided options with a preference over inferred ones.

- closes #110

- TODO: crashes `vim -g -u test/vimrc`

This reverts commit bb402facfb.

- previous commit closes #13

- closes #21