Rewrite upstream PR #49: safe configurable fzf overrides

[xxxbrian]

Summary

• rework the useful parts of upstream PR Allow override of rm and custom fzf options gazorby/fifc#49 on top of this fork
• add configurable temp-file cleanup via fifc_rm_cmd
• support global custom fzf options without mutating the user's universal config at runtime

Why rewrite instead of mirroring directly

• the original PR mixes the keybinding change with config overrides
• this fork is importing the keybinding cleanup separately from upstream PR fix: remove extra tab binding gazorby/fifc#61
• this rewrite keeps fifc_custom_fzf_opts as user config and uses a separate effective runtime variable during completion

Source

• upstream PR: Allow override of rm and custom fzf options gazorby/fifc#49