allow -x to pass on the expose option

Similar to how you can use -r ./module-file.js:module-name to set the
exposed require name for a module, we should be able to set the exposed
module name for external modules.

This allows the user to require vendor libraries into a bundle using
short names, and to allow access to those libraries using the short
name.

An example using jquery installed with bower might look like:

browserify -r ./bower_components/jquery/jquery.js:jquery -o vendor.js
browserify -x ./bower_components/jquery/jquery.js:jquery app.js
Added cacheify to the list of known transforms in the README
Update readme.markdown

add folderify row to transforms
Update readme.markdown

boops
Update readme.markdown

beeps
transform formatting
minor readme help syntax fix
Added phantomjsify to README
formatting fix
Provide example using the optional target syntax

Took a while for me to actually figure out how the optional target worked and thought it would be handy to provide an example in the readme.
3.6.0