Revises --fuzzers argument docs

[bwolfe-corvus]

• If used with no arguments, prints available fuzzers
• default fuzzer list matches with generate() function's possible defaults

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Resubmitting with a simpler change. You were right in that I did not originally notice that the various and tld-swap fuzzers weren't tied to class methods. Since I'm only looking to do a change that affects ease of usage, I just tried to match up the --fuzzers argument with how the default fuzzers are used within the generate() function as a preset list.

Original output sample:

$ python dnstwist.py --fuzzers --format list contoso.com
usage: dnstwist.py [OPTION]... DOMAIN
dnstwist.py: error: argument --fuzzers: expected one argument

New output:

$ python dnstwist.py --fuzzers --format list contoso.com
usage: dnstwist.py [OPTION]... DOMAIN
dnstwist.py: error: argument --fuzzers requires a comma-separated list of fuzzers.
Available fuzzers: addition, bitsquatting, cyrillic, homoglyph, hyphenation, insertion, omission, repetition, replacement, subdomain, transposition, vowel-swap, dictionary, tld-swap, various

I intentionally did not wrap the docstring error output to a specific width, as I didn't see that as a behavior in other parts of the code. However, I found that Python's stdlib has a textwrap module going back to 2.7, so if that's something of interest, I will wrap this output appropriately.

Forgive the double PR, I blew up my forked repo and had to resubmit a PR from a clean state.