Allow hints to be placed anywhere in query

Allow hints to be placed anywhere in query

Hints description is restriected to be placed before certain
characters. That is effectively at the beginning of a query, or after
EXPLAIN or PREPARE.  This commit adds a new setting parameter
pg_hint_plan.hints_anywhere, which gets rid of that restriction.  When
it is on, pg_hint_plan ignores SQL syntax at all while reading hints
so there's no restricion on where hint string is placed in a query
string. On the other hand it may lead to false reads from a non-hint
strings.