Add $FZF_DEFAULT_OPTS_FILE

[junegunn]

For those who prefer to manage default options in a file. If the file is not found, fzf will exit with an error.

We're not setting a default value for it because:

1. it's hard to find a default value that can be universally agreed upon
2. to avoid fzf having to check for the existence of the file even when it's not used

Comparison to the alternatives

1. Reading the file from the shell configuration file

export FZF_DEFAULT_OPTS=$(cat ~/.fzfrc)

Compared to the above, with FZF_DEFAULT_OPTS_FILE,

• You don't have to reload your shell configuration file after editing the file
• An external script can modify the configuration file in the background

2. Wrapper script or function

fzf() {
  FZF_DEFAULT_OPTS=$(cat ~/.fzfrc) command fzf "$@"
}

This gives the same benefits of FZF_DEFAULT_OPTS_FILE listed above. But you have to manage the function or script yourself.