Expand cljr magic require namespaces documentation

Minimal documentation on the :only keyword.
Backport support for :only in the cljr-magic-requires-namespace if the suggest-libspec flag is off
Adds edn alias and limits io alias to clj only

Still needs more testing and expanded documentation but this seemed like a good incremental improvement.

Before submitting a PR make sure the following things have been done (and denote this by checking the relevant checkboxes):

[x] The commits are consistent with our contribution guidelines
[x] You've added tests (if possible) to cover your change(s)
- not adding test coverage here as this is the deprecated path, but I did verify the parser change at the elisp repl.
[x] The new code is not generating byte compile warnings (run cask exec emacs -batch -Q -L . -eval "(progn (setq byte-compile-error-on-warn t) (batch-byte-compile))" clj-refactor.el)
[x] All tests are passing (run ./run-tests.sh)
[x] You've updated the changelog (if adding/changing user-visible functionality)
[x] You've updated the readme (if adding/changing user-visible functionality)

Thanks!

clojure-emacs / clj-refactor.el