TS-1823: add line continuation to remap.config

Add line continuation to remap.config using a backslash at the end
of a line as the continuation character.

The handling for continuation was made in tokLine() by adding an
optional third char parameter (`cont') which is the continuation
character. This should make it usable in other contexts outside of
remap.config also, for example other config file parsers.

This implementation is not very intelligent, as it only checks for the
backslash immediately preceding the newline and does not handle any
whitespace. The backslash and newline are converted into spaces, and
the next line is appended.

Example:

.definefilter foo \
  @action=allow \
  @src_ip=127.0.0.1

is read and parsed in UrlRewrite::BuildTable() as:

.definefilter foo     @action=allow     @src_ip=127.0.0.1
TS-1597: Document remap.config filters

Add documentation and examples for the
.definefilter, .activatefilter, and .deactivatefilter
directives in remap.config.

These docs are specifically restricted to these particular names
because (1) we should standardize on one variant for each of these
directives to avoid end-user confusion and (2) I personally believe
these variants to be the clearest and most understandable ("defflt",
"useflt", "deuseflt", etc., are confusing or awkward).

The .deletefilter directive is not documented, because I am not sure
when or why it would be used.
Fixed changes from merge.