Dynamic "compiler" format (a.k.a. "ad-hoc dynamic") requires --format option

The dynamic compiler format should be recognized without a --format option when given a format tag.

Example:

$ echo > test.in '@dynamic=md5($p.$u)@16ccb184ee1f16dd9af509135f504442$$Ujumbo'

$ ../run/john test.in
Using default input encoding: UTF-8
No password hashes loaded (see FAQ)

$ ../run/john test.in -form:'dynamic=md5($p.$u)'
Loaded 1 password hash (dynamic=md5($p.$u) [256/256 AVX2 8x3])
Press 'q' or Ctrl-C to abort, almost any other key for status
magnum           (?)
1g 0:00:00:00 DONE 2/3 (2018-01-24 11:43) 2.631g/s 8842p/s 8842c/s 8842C/s 123456..Geronimo
Use the "--show" option to display all of the cracked passwords reliably
Session completed

This behavior/requirement is documented, but if possible we should enhance it.

One effect of this problem is that external tools can't get any info using --show=types[-json]

$ ../run/john test.in --show=types
?:@dynamic=md5($p.$u)@16ccb184ee1f16dd9af509135f504442$$Ujumbo:::/:/:/:0: