Multi shell support for task execution

[rdsubhas]

Right now, we invoke all tasks with /bin/sh. This thread is to discuss if and how to support multiple shells (example, cmd.exe, powershell, bash, zsh, etc). Options to be considered:

• YML syntax
• Multi-shell and/or multi-platform. e.g. Windows can use both /bin/sh or cmd.exe, Linux could use /bin/sh or /bin/bash, etc. So shells and platforms are not 1:1 but rather cross functional
• myke specific feature compatibility: environment variable cascading, retry and exit codes, verbosity levels and stdout/stderr handling, etc

Refers #85