major cosmetics (interface change) + bugfix of 0.8.3

Omd_parser.parse in Omd_parser  have been moved to a  functor in order
to expose main_loop (and main_loop_rev) and to keep reentrance for the
users of the library.

In  order  not  to  impose  the  use  of  the  functor,  the  function
Omd_parser.default_parse   is    provided   as   a    replacement   of
Omd_parser.parse,  which  no longer  exists.  This renaming is  mainly
motivated  by the  fact that  there will  eventually be  many  ways to
instanciate the  parser (notably with extensions  and link definitions
environment) and the name should reflect this. ("will eventually be"
because the extension mechanism is not very mature yet)

The other files (omd.ml{,i} and  omd_main.ml) have been adapted to use
the new interface of Omd_parser.
(remove spaces at end of lines)
add test (blockquote-starting-with-empty-lines)
change semantics of blockquotes

2 blockquotes separated by an empty line are considered one blockquote.
If they are separated by more than a single empty line, they are considered
separate blockquotes.
tests for blockquotes fusion
prepare the ground for code parsing in read_until_...
fix typo
move most top-level values of Omd_parser to its functor Make

Omd_parser.Make  is functor  that  takes an  environment (module  type
Omd_parser.Env)  and  returns  a  markdown  parser  that  shares  some
environment values.  This allowed to replace some top-level references
(of Omd_parser) by values passed in the module parameter (Env).

Files concerned by this change:
 - omd.ml
 - omd_main.ml
 - omd_parser.ml
 - omd_parser.mli

Also, omd_parser.ml has been entirely re-indented.
update comments
update interface
expose values in Make that come from its parameter Env
fix markdown output for blockquotes
fix: multi-part blockquotes

Now
<<EOF
> 1

> 2
EOF
is equivalent to
<<EOF
> 1
>
> 2
EOF
and they mean a blockquote with 2 paragraphs,

but
<<EOF
> 1

> 2
EOF
means 2 separate blockquotes.
semantics change: multi-part blockquotes
fix multipart blockquotes
remove NL after paragraphs in markdown output
functorise Omd_parser.Default_env

because otherwise there may be messy shared values
(in particular the reference environment)
avoid escaping dots when not preceded by a 0..9 digit
fix links that carry blockquotes

Omd_parser.maybe_link has been rewritten.
fix endless loop (typo) in read_until_...