Vundle manipulates plugin folders by `cd`-ing into them and calling git
via `call system(cmd)`. This doesn't work if GIT_DIR is set as a global
environmental variable (most commonly by a git hook). Since there is no
use case in which Vundle should respect GIT_DIR, this PR preemptively
precedes all git calls with `unset GIT_DIR; `. This makes the git calls
behave as expected without unsetting GIT_DIR in the environment beyond
vim.

* Add modelines to all code/doc files excluding README.md.  The modeline used is: `" vim: set expandtab sts=2 ts=2 sw=2 tw=78 norl:`
* Rename links with /vundle to /Vundle.vim, seems to confuse some people with redirect.
* Remove duplicates from Quickstart, add example of `name` flag.
* Add link to Tips page by @Lucc .
* Change help maintain vundle link directly to the new one.
* README.md line number links updated.

Note: Users with bundle/vundle need to rename vundle to Vundle.vim to use test files. Vundle will continue to function otherwise.

Fixes #413.
Fixes #438.

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