Check if SHELL supports FZF_DEFAULT_COMMAND

[thyssentishman]

When FZF_DEFAULT_COMMAND is unset, it defaults to a command that depends on 'set -o pipefail'. Since not every shell supports this option, bash was hardcoded as the default shell to be used for this specific case. With this commit, other shells that support this option are first compared to SHELL before depending/falling back to bash.

This commit closes #3339

[thyssentishman]

@junegunn here is an attempt at implementing your suggestion in #3339. I'm new to this codebase and go in general, so apologies in advance for any possible mispractice.

[junegunn]

Thanks, I have had an almost identical patch locally, so let me finish this up.

[thyssentishman]

Awesome, thank you very much!