Add RANGE with values and exclusions clauses to the Window Functions

Adds RANGE BETWEEN with a start and end value, as well as an
exclusions clause, to the window functions. This partially resolves
TODO list item "Implement full support for window framing clauses".

== Specification ==

The window functions already allow a "ROWS BETWEEN start_value
PRECEDING/FOLLOWING AND end_value PRECEDING/FOLLOWING" to restrict the
number of rows within a partition that are piped into an aggregate
function based on their position before or after the current row. This
patch adds an equivalent for RANGE which restricts the rows based on
whether the _values_ of the ORDER BY column for all other rows in the
partition are within the start_value and end_value bounds. This brings
PostgreSQL to parity with Oracle, and implements a SQL:2011 standard
feature.

SQL:2011 also defines a window frame exclusion clause, which excludes
certain rows from the result. This clause doesn't seem to be
implemented in any mainstream RDBMS (MariaDb mentions that fact in its
documentation here:
https://mariadb.com/kb/en/library/window-functions-overview/ and has
it on its TODO list). This patch implements three EXCLUDE clauses
described in the standard:

EXCLUDE CURRENT ROW - excludes the current row from the result
EXCLUDE TIES - excludes identical rows from the result
EXCLUDE NO OTHERS - does nothing, is the default behavior; exists
purely to describe the intention not to exclude any other rows

The RANGE BETWEEN clause requires a single ORDER BY column which must
be either an integer or a date/time type. If the column is a date/time
type then start_value and end_value must both be an interval type. If
the column is an integer, then the values must both be integers.

== Testing ==

Tested on Windows with MinGW. All existing regression tests pass. New
tests and updated documentation is included. Tests show both the new
RANGE with values working and the exclusion clause working in both
RANGE and ROWS mode.

== Future Work ==

The standard also defines, in addition to RANGE and ROWS, a GROUPS
option with a corresponding EXCLUDE GROUP option. This also doesn't
seem to be implemented anywhere else, and I plan to implement it next.

This patch also adds some new error messages which have not been
internationalized.