Update for more recent golang and add variable-length separator characters

[icanhazpython]

I have done a few preferable (in my opinion) updates here:

• Updated project for go v1.18; direct and indirect dependencies have been updated and the vendoring directory has been removed
• Switched from packr to go:embed for words list
• Added variable-length separator setting; this can yield passwords such as (see example config below):
  • clubs0344driving
  • history8330chassis

Example config for above passwords:

{
  "num_words": 2,
  "separator_alphabet": ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"],
  "padding_characters_before": 0,
  "padding_characters_after": 0,
  "case_transform": "LOWER",
  "padding_digits_before": 0,
  "padding_digits_after": 0,
  "separator_characters": 4,
  "separator_character": "RANDOM"
}