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This allows for the user to combine the jump command with something else. In my example cd and jump are now combined like this:

```bash
jumpcd() {
        jump $1 > /dev/null || cd $1
}
alias cd="jumpcd"
```

This avoids spawning additional processes as $PWD **always** contains
the current working directory.

This change fixes the autocompletion for marks which contain numbers or dots.

Fixes #2578

`\d` in sed regular expressions doesn't work (see http://stackoverflow.com/questions/14671293/why-does-d-doesnt-work-in-regular-expression-in-sed)

Autocompletion fails if there's only one mark, since the ls command
will not display the parent directory with the trailing colon.
Handling the single mark case separately and validating the symlink
explicitly, resolves the issue.

Change marks function and remove 'function' keyword as suggested by pielgrzym in https://github.com/robbyrussell/oh-my-zsh/pull/2045#issuecomment-22820224

Add _completemarks function as suggested by pielgrzym in https://github.com/robbyrussell/oh-my-zsh/pull/2045#issuecomment-22826540