This patch implements 'isearch-yank-until-char', a new yank command in isearch.

While in an isearch, one types C-M-c to yank into the search string everything from point up to (but not including) the next instance of a specified character.  It prompts for the character.

For example, assume point is on the "m" after the "#" mark below:

  [example from a recent macro I wrote](#markdown-link-syntax)

Start an isearch with C-s, then do C-M-c followed by ")".  The search string is now "markdown-link-syntax".

I have found this functionality very helpful in macros.

If it's a useful contribution to Emacs, then I'll install it.  I'll wait at least three days to get feedback, both about the feature itself and about the choice of M-C-c as the keybinding.  If there's no response or no consensus, then I won't install it, as being conservative about isearch seems appropriate.

Best regards,
-Karl