install dep plugins & run provider hooks the same as shell hooks (Rereopen 316)

Fix ns help

modify `ct` provider to copy selected directories and compile them

alongside their source (ie, if `some_tests` is a directory that
contains test suites beams resulting from compiling them will be
placed in `some_tests` in the appropriate `_build` directory

update the dir used for upgrade deps to the new path under _build

impl shell completion

main module of a plugin just has to export init/1

Support namespace resolution in 'do' provider

cover enhancements

Fixes #306: help text consistency

fix clean all to use proper deps dir

re-export functions only used within rebar_utils that are also needed by...

fix congratulation message in README.md

Fix some dialyzer warnings

generate line by line coverage reports

Polish rough edges of 'new' provider

- Allow 'rebar3 help' to work and have it point to 'rebar3 help
  <template>'
- show a 'template not found' message when a template is indeed not
  found rather than crashing.

to the appropriate `_build` dir

eunit refactor and test suite

matching directory paths

run subsets of test suites

test utils and add better eunit tests

Fix property merging

Much clearer semantics now. All lists are treated as proplists, meaning
we want to:
1) allow duplicates (providers have to avoid them if they must)
2) preserve order of elements that compare equal (`a == {a, val}`)
   through a stable sort (so if `{a, b}` comes before `a`, we keep
   `{a, b}` first in the list
3) In two lists of attributes requiring a merge, we always give the
   'new' profile a priority to override the default one.