{port_specs, [{".*", "priv/foo.so", ["c_src/foo.c"], [{env, []}]}]}.

{pre_hooks,  [{"linux", compile, "c_src/build_linux.sh"}]}.
{post_hooks, [{"linux", compile, "c_src/build_linux.sh clean"}]}.

This patch modifies rebar_core to allow plugins to participate in the
pre and post processing steps, giving plugin authors more flexibility
and control.

Avoid overwriting custom config (passed with -C) when we are processing
the base_dir in rebar_core.

This patch fixes the warning logging when the number of missing plugins
is greater than one. The current code only works by accident, when a
single plugin is all that is missing.

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.

This patch makes a small change in rebar_core that checks the list
of valid plugins to see if any of them export a pre/post processing
function for the current command. This logic is applied only to the
plugins and allows plugin authors to hook into rebar's execution by
using a naming convention that matches the one used for scripting hooks.

Example:

```erlang
-module(my_rebar_plugin).
-export([pre_compile/2]).
pre_compile(Config, AppFile) ->
    rebar_log:log(debug, "PRECOMPILE: ~p:~p~n",
                  [AppFile, Config]),
    ok.
```

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.

Rebar will exit with {error,bad_directory} when trying to restore the code
path after it has finished working on a subdirectory if there are invalid
relative paths in it. The problem was seen when executing the last line of
rebar_erlc_compiler:doterl_compile/3 (true = code:set_path(CurrPath)).

I have moved these macros from rebar_core.erl to rebar.erl
in order to prevent eunit tests from failing (which they
currently are).

This change makes it possible to type the beginning (the prefix) of a
command name and rebar will guess the full name of the command,
thereby saving the user precious keystrokes.  As long as the prefix
matches only one command, rebar runs that command, otherwise rebar
prints a list of candidate command names. The "-" character is
considered to be a word separator and the prefix matching is done per
word.

Example prefix matches:

    co       ==> compile
    cl       ==> clean
    create   ==> create
    create-a ==> create-app
    c-a      ==> create-app
    c-app    ==> create-app

Make the error message newline terminated and, if possible, log the name
of the module in which the failure occured.