New rank tweaks: follow, lead, length

New rank tweaks: follow, lead, length

Rank tweak: follow=number will increase mult by number if two terms
occur next to each other; number-1 if they are one term apart , .. 0
if they are number a part (all in order). Default is 0 (following
terms has no effect).

Rank tweak: lead=number will increase mult by number if term is first
term in field, number-1 if second, ... 0 if term is at offset
number of more. Default value is 0 (position irrelevant).

Rank tweak: length=strategy. length="linear" if mult is to be divided
by length (existing, default behavior), length="log" if mult is to be
divided by log2(1+length), length="none" if mult is not to be affected
by length.