bug#10885: Replace expressions: enhance functionality when searching in filled paragraphs

[Juri Linkov]

After installing
http://lists.gnu.org/archive/html/emacs-devel/2012-09/msg00008.html
this functionality is available to query-replace, but currently only
when it's started from isearch.

To use it in query-replace independently from isearch,
I see two ways to toggle whitespace matching in query-replace:

1. Add a new defcustom like `case-replace' with a name e.g.
`replace-lax-whitespace' and use it in `perform-replace'.
When non-nil, spaces will match any whitespace in replacement.

2. Add a new command like `query-replace-regexp' with a name e.g.
`query-replace-lax-whitespace', where to let-bind
`replace-lax-whitespace' to t and call `perform-replace'.