Add POSIX compatibility

[jkroepke]

it's almost a rewrite of the shell script for helm script.

I remove all bash related shell code and replace it with POSIX feature. The script will run on almost any posix compatible shell. The scripts is also compliant to shellcheck.

ZSH (MacOS default shell) and ash (Alpine default shell) should now out of the box supported. gnu-getops is not required anymore.

All tests from test.sh are green under MacOS 10.15.4 (zsh, bash 3.2.57) and Alpine 3.11 (ash)

Additional features:

All helm commands are supported now. Before this only install|template|upgrade|lint|diff are supported. If plugins like kubeval are installed this script need to be modified. The PR remove the limitation.

Fixes:

• helm_wrapper does a verbose rm for cleanup interfering with templating zendesk/helm-secrets#126 (verbose output redirected to stderr)
• Support secrets[._-]*.yaml zendesk/helm-secrets#128 (name of secret more relaxed)
• Install SOPS without root zendesk/helm-secrets#141 (dpkg variant dropped; you should never run helm plugin install as root)
• Removing verbose deletion as it breaks helm template functions. zendesk/helm-secrets#149
• Fix tab zendesk/helm-secrets#150
• Support more file name and output what is decrypted zendesk/helm-secrets#129 (All name restrictions dropped)
• add support for helm-kubeval zendesk/helm-secrets#143
• Install SOPS without root zendesk/helm-secrets#142 (dpkg variant dropped; you should never run helm plugin install as root)
• Fix template command to not print list of deleted files zendesk/helm-secrets#136 (verbose output is not on stderr)