It was hard to separate this too fixes (same regexps).

Namely monadic notations and Equations (the latter is bugged
currently).

This a generalization of PR#451 proposed by @Chobbes.

Since these syntax are not completely universal (and not builtin in
coq), the idea is to make the syntax customizable, especially to make
it possible to disable it.

NOTE: to make the Compcert syntax supported I had to refine the smie
lexer so that the ";" token was emitted as a fllback instead of ";
tactic".

NOTE2: I had to make the coq-user-token and coq-monadic-tokens be
tested ON THE RESULT of smie-coq-backward-token.

Fix indentation

While fixing these I discovered several flaws in indentation (what a
suprise). The probem is the following: since we don't have a precise
grammar of tactics, smie often decides that the parent of a sub-part
of a tactic is the previous command instead of the current tacic.

Typical example (fixed now but in general):

             foo.
  apply bar
             with bar'.

Since "apply ... bar'" is considered as one leaf of the grammar, it is
considered to be a child of the previous dot.

     .
     /\
    /  \
  foo  apply...bar'

Therefore indentation of "with" wants to align with parent "." and
hence with "foo".

Basically we should try to define a much more precise grammar of
complex tactics (with with, as, using etc) in order to fix the
problem. Of course this has the drawback of making impossible to
support user tactic notations.