Replace various language support plugins with vim-polyglot

I also remove my Clojure file recognition autocommand because that is included within the Clojure support of vim-polyglot.

My “vim-markdown” plugin was a different one from the “vim-markdown” plugin that vim-polyglot currently uses, but I don’t mind the switch, because the new one is by tpope, who is reputable, and I remember a persistent bug with empty `` code blocks that I hope the new plugin fixes.
Add plugin ack.vim
Update all plugins

Two of the plugins, honza/snipmate-snippets and kogakure/vim-sparkup, were gone, but I found the the honza one had merely moved, and there was a similarly new vim-sparkup replacement.

Note that Vundle has been updated too. It suggests using Plugin instead of Bundle, which I’ll probably do.
Correct the labeling of file extension autocommands

To reflect that I removed the Clojure autocommand because vim-polyglot already included an equivalent
For Vundle, switch from Bundle to new Plugin command

Also switch to the more standard formulation of “filetype plugin indent on” – this ordering seems to be the one always cited in plugin READMEs
Add vim-airline plugin

And add a comment to remove the trailing whitespace warning it gives on my vimrc
Remove '>' and '<' separators in Airline (status line)

They don’t look that good without a custom font, and the line seems just as readable without them.

I tried it because it was recommended on http://sheerun.net/2014/03/21/how-to-boost-your-vim-productivity/
Add scriptease.vim and splitjoin.vim

Scriptease will make editing my vimrc easier with ‘K’, and let me debug the new Markdown plugin’s (from vim-polyglot) non-highlighting of code

Splitjoin would have been useful when I was working with XML a lot. I might need it again. It was recommended in http://sheerun.net/2014/03/21/how-to-boost-your-vim-productivity/ .