This patch adds support for short options in the eunit provider.

To prevent future mistakes like the broken string introduced in
cf66dfd6ba, put opening and closing " on separate lines in each
multi-line lopts string. This also allows for cleaner (smaller) diffs
when removing/adding a flag.

cf66dfd6ba introduced a syntax error in the modified lopts string.
Without the fix, you'll provoke a shell error when trying to complete
`rebar3 ct --<something>`.

Always-on recursive application of all rebar commands causes too many
issues. Recursive application is required for:
1. dealing with dependencies: get-deps, update-deps, and compile of deps
   right after get-deps or update-deps
2. projects with a riak-like apps/ project structure and dev process

The vast majority of projects are not structured like riak. Therefore,
moving forward it's best to (by default) restrict recursive behavior to
dealing with deps. This commit does that and also adds command line and
rebar.config options for controlling or configuring recursion. Also, we
introduce two meta commands: prepare-deps (equivalent to rebar -r
get-deps compile) and refresh-deps (equivalent to rebar -r update-deps
compile). riak-like projects can extend the list of recursive commands
(to include 'eunit' and 'compile') by adding
{recursive_cmds, [eunit, compile]} to rebar.config.

- add overlay_vars=, target_dir=, and tests= with short description

- add missing short and long options
- add tests=, target_dir=, overlay_vars=

- Add 'escriptize' command
- Remove variable 'dump_spec'

For enable to shell-completion in zsh.

This change adds the ability to use alternate urls for downloading
dependencies. To make use of alternate urls run:
rebar get-deps alt_urls=true

Add no_debug_info option.
Remove debug_info=1 option.

Add generate-upgrade and previous_release=.