Add xref support for multi-app projects. Instead of only adding the
top-level ebin directory to the xref server use
rebar_state:project_apps/1 to enumerate the applications for the
project and add the ebin directory for each one.

* Add a provider for the xref tool for running cross reference
  analysis on a project. Most of the code has been ported directly
  from the rebar2 rebar_xref module with some modification and cleanup
  to support testing.
* Port over the eunit test suite from rebar2, but convert it to
  common_test. The testing is the same, but now the
  erlang term output is examined to determine if the test run is successful
  instead of scanning the console output for the expected strings.

(in `rebar_file_utils`)

`erl_first_files`

fixes #99

from run to run

`common_test` and `eunit` providers

option handling

in presence of keys defined multiple times

compiles common test files in a temporary dir (or a user specified
dir) and runs tests without leaving artifacts in the source tree

precompile tests using `rebar.config` and disable autocompile so
project specific options are used

fixes #62

compiles apps and eunit test files in a temporary dir (or a user
specified dir) and runs tests without leaving artifacts in the
source tree

automatically defines the 'TEST' macro if not already defined and
adds `test` to `src_dirs` to allow for eunit test files outside `src`

compiled first

- Includes tests for templates and their application

Now supports arbitrary commands

Using a namespace such as 'rebar3 lfe new' will look for the 'new'
command in 'lfe' namespace without the need for a 'do' provider to be
registered in that namespace.

Manually checked that 'rebar3 as <profile> <namespace> command' works,
as well with 'default' and 'do'.

Test suite still needed.