Introduced draft of SSG Bash scripting guidelines.

[matejak]

As @OnceUponALoop in #2617 noticed, it is not clear what style and approach to prefer when contributing to Bash remediations.
I have went to the existing remediations in the shared/fixes/bash folder and observed the prevalent existing style. I have found out two points which I believe that are not controversial at all:

• most scripts use tabs for indentation, so let's use tabs.
• usage of sed greatly surpasses awk, so let's encourage sed.

Next, I propose these:

• Use the die function idiom to handle possible errors (I have defined it and checked that it can be actually used).
• Run shellcheck over the snippet and try to fix all warnings.
• Avoid double negatives s.a. if ! [ $x != 1 ]; then ...

Finally, some points may be controversial:

• I have slightly encouraged usage of compound statements. I think that if they are used in moderation, they can make the code much shorter, without obscuring anything.
• I have suggested to write scripts in a way that they work if invoked in the "strict mode" e.g. by running them using bash -e or by writing set -e to the top of the script. This imposes greater requirements on error handling inside the script and therefore increases its robustness.

[mpreisler]

ACK, great stuff!

[shawndwells]

Agreed, thank you for doing this @matejak!