Improvements to capsule

[XuJiandong]

1. Docker's network mode can be switched to "host" by flags "--host".

It will make configuring proxy easier. Also update the wiki: Proxy-Configuration when host mode is on.

2. Pass arguments to cargo build

All arguments followed by "--" can be passed to cargo inside docker while using capsule build.

3. Make "capsule test" deprecated
   Print warning when users use "capsule test":

Capsule test subcommand is deprecated. Use `cargo test -p <...>` instead.

4. Pass environment variables in command line cargo build

--env-file list                  Read in a file of environment variables

It will be used by docker directly.

[TheWaWaR]

Implemented in #57