Add integer base support to inspect

Option :base for inspect/2 supports :binary, :octal, :decimal, :hex.
Includes tests and documentation.
Merge pull request #2627 from smpallen99/inspect_base

Add integer base support to inspect
Since IO.ANSI.enabled? is a fast operation, no need to prefetch it on IEx
Remove documentation from defimpls
Update CHANGELOG
Load mix deps only when there is a need to use them

This helps speed up peformance to many tasks. Tasks like
clean, help and compile --list must now explicitly call
loadpaths with checks disabled so they can still show
data coming from dependencies but without causing crashes.
Update Mix.Dep.Lock documentation
Do not expand args for unknown functions, closes #2387
Use the Logger PID to avoid race conditions on shutdown, closes #2628
Doc correction for GenEvent
Merge pull request #2630 from scrogson/patch-2

Doc correction for GenEvent
Fix docs and specs introspection in iex
Merge Logger.metadata
Fix iex test
align the messages between log messages with different length level text. Added an extra format $levelpad, which is a space if the level is "warn" or "info"

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>
Support  on log messages
Remove Windows-related CONTRIBUTING.md section (now in Wiki)
Merge pull request #2631 from chyndman/contributing-windows

Remove Windows-related CONTRIBUTING.md section (now in Wiki)
Use `not` instead of `!` in `IO.ANSI.enabled?/0`
Merge pull request #2633 from lexmag/not-match

Use `not` instead of `!` in `IO.ANSI.enabled?/0`