Add new command `prelude-top-join-line`

Saner version of the built-in `join-line`, that joins lines from the top
to the bottom.
Recompile the Prelude dir on `prelude-update`
Vanity URL for the installer

Made with
`https://github.com/bbatsov/prelude/raw/master/utils/installer.sh`

More info: https://github.com/bbatsov/prelude/raw/master/utils/installer.sh
Use valid shell commands

By not prepending `$` to your examples users can copy them easier
without fear of seeing `bash: $: command not found` or similar
Merge pull request #333 from joshuarh/vanity-url

Vanity URL for the installer
Merge pull request #334 from joshuarh/valid-shell-commands

Valid shell commands
Bind prelude-top-join-line to C-^
Merge branch 'master' of github.com:bbatsov/prelude

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