[feat] POSIX compliant

[simonmysun]

As mentioned in #30

These bash features are waiting to be rewritten with POSIX compliant shell:

• source
• function name with ::
• [[
• echo -ne
• string indexing
• export -f (except in test)
• here-string
• process substitution
• +=
• access to BASH_VERSINFO
• access to PIPESTATUS
• array references
• declare -p -x
• declare -f
• read -N 1
• $'..' (already accepted? https://austingroupbugs.net/view.php?id=249 )
• read -n 1 -s -r -p "Press any key to continue" < /dev/tty;