Added a "latest.markdown" symlink to the latest post, to save typing.

I've modified the new_post function so that it automatically creates a symlink to the new post called "latest.markdown". This means that instead of typing 

    new_post[post1]
    vim 2013-04-30-post1.markdown

you can instead type

    new_post[post1]
    vim latest.markdown

If you are used to using tab to autocomplete file names in bash, you will realise that all you really need to type is 

    vim l<tab>

because this will be the only file starting with "l" in the directory.

This is much quicker, especially when there are multiple posts in the same year, month and day.