`cover` task

`as` delegates task handling to `do` provider

pre-purging before checking to facilitate long do chains

source files in the `compile` key of `module_info/1` so `rebar cover`
struggles locating source in some instances. in these cases a warning
is printed and crappy coverage info is written rather than none at all

`rebar.config` or via the `-c\--cover` flag given to the appropriate
task) from the `eunit` and `ct` tasks and add a `cover` task to
write coverage analysis to disk

make base_dir for a run include the profiles in path, link to shared dep

Some free tests from a debugging session

The session was unfruitful, but I guess the tests were not necessarily

don't replace non-string list with empty list in profile merge

Documentation is maintained on www.rebar3.org

reset state between tasks run with `do`

has the same outcome as `rebar3 foo && rebar3 bar && rebar3 baz`

fixes #191

Port env var expension fix from rebar2

add edoc provider

Support mercurial (hg) dependencies

See https://github.com/rebar/rebar/pull/458

remove old info function from install_deps, it is a bare provider

Fix crash on non-git source deps in deps provider

The 'element(1, ..)' guard would always fail and cause crashes.

profiles: combine lists of values for conflicting config options

We can use 3.0.0 when we're done.

Upgrade pkg

- Track level of packages properly, they're not level 0 anymore (this
  could yield an issue where a src dep takes precedence over a pkg dep)
- Proper stable sort of vertices in the digraph module
- PkgDeps no longer 'see themselves' when fetching and upgrading after
  locking themselves
- Pkg Locks are added to pkg deps rather than source deps
- Updating test cases to support pkg mocking on top of src mocking

Fix lock merging for nonexistant lock files.