Allow recursive project searches to add a directory explicitly

[mohkale]

See #1500.

Now you can add a directory DIR to projectile-project-search-path with
depth 0 to mean that if DIR is a project then add it to projectiles
project list.

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  This was already taken care of by [Fix #1500] Add support for recursive project discovery #1680.
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Thanks!

[bbatsov]

Thanks!