Add proxy support to bootstrap and rebar3. Enhancement #561.

Changed cases on robocopy so it uses variables in a better way

Robocopy fix for different behaviour when copying/moving files and folders

Run agent as current process & hibernate

This tries to reduce memory usage when running `rebar3 shell` by running
the agent in the current process (and avoiding to copy state
cross-boundaries), and using frequent hibernation after each run to
force a full GC and compaction of the current process.

wip: deps install refactor

Minor refactors

remove `cover_print_enabled` and `cover_data_dir` config options and replace with `cover_opts`

Apply profiles properly & load all paths

and replace with `cover_opts`

The old run didn't necessarily apply all profiles well, and would only
reload modules in the app file. However, since adding extra_src_dirs,
modules can be compiled without ending up in the app file; this lets
the rebar3 shell agent handle that case.

update plugin template to separate plugin init and providers

add hex attributes to .app.src templates

Read and process .app.src.script if present

setup hex httpc profile and http opts. needed also for proxy

Oops, this didn't get in

update contributing, readme and app.src for beta-1

only reverse some relx args, fixes profile usage for relx

Revert "disable ctrl-c interrupt in shell for escript"

This reverts commit 40ee6d95e1.

disable ctrl-c interrupt in shell for escript

Make test on windows green.