This patch updates rebar_core to look for missing plugins (i.e. those
that aren't found on the code path at runtime) in a configurable
plugin directory, and dynamically compile and load them at runtime.

By default, the directory "plugins" is searched, although this can be
overriden by setting the plugin_dir in your rebar.config.

Change the second parameter of a regex tagged port_source from being a
filename or wildcard to being a list of filenames or wildcards.

Previously: {"R14", "c_src/*.c"}
Now: {"R14", ["c_src/*.c"]}

Motivation for change is to avoid repeating regexes.

This patch adds a `ct_extra_params` option to rebar.config, the value
of which is appended to the shell command when executing common test.

Add no_debug_info option.
Remove debug_info=1 option.

This change makes it possible to assign pre/post scripts to all
rebar commands. This allows users fine grained control over
when scripts and/or shell commands should be executed, where such
extensions are absolutely needed.

Several examples have been added to the rebar.config.sample file.

Add a coverage report similar to the one output to index.html except
that it is output to the terminal if the new rebar.conf option
'cover_print_enabled' is set to true.