Add 'extend' capability for tasks

[sagiegurari]

Features Description
Add a new 'extend' attribute for tasks to enable it to extend any other task.
This will enable to create a 'template' task from which actual implementations with few changes extend from.

[sagiegurari]

#201

[ssokolow]

You'll want to document that an explicit private = false is necessary in this situation:

[config.modify_core_tasks]
private = true
namespace = "default"

[tasks.kcov]
extend="default::coverage-kcov"
private = false

If someone's used to thinking in terms of object-oriented programming, it probably won't occur to them that you can change something's public/private status when extending it, they'll run into this error, and then mistakenly conclude that it's impossible:

[cargo-make] INFO - Using Build File: Makefile.toml
[cargo-make] INFO - Task: kcov
[cargo-make] INFO - Profile: development
[cargo-make] ERROR - Task kcov is private
[cargo-make] WARN - Build Failed.

I was actually caught by that assumption for a moment.

[sagiegurari]

I'll add this to the docs.