:s fixes/tweaks

[Shadowfiend]

I'm going to generally follow a policy of keeping a pull request open for my own stuff for at least a week to gather any feedback the community may want to give for now; this can evolve as the community grows. If I open a pull request and don't see any activity for a week, I'll go ahead and merge it in. Likewise, I'll try to give feedback on any requests I see within a week if at all possible.

This PR does a variety of things with the :s/:substitute command, and fixes a couple of incidental issues because I ran into them as I was making these changes. Here's a list:

• Fixes undo behavior for :s. Before undo would undo each substitution in turn, whereas we now properly undo all substitutions at once. This is more in line with vim behavior.
• Fixes caret behavior for :s. At the end of a substitution, the caret is now properly placed at the end of the last match that was substituted, or at the end of the file if nothing matched.
• Fixes jumplist tracking for :s. After a substitution, the previous location in the jumplist is now the place where you were when you ran the substitution command. This means immediately after a substitution command, you can use '' or its sibling `` to return to the point you were at before you performed the substitution.
• Fixes newline handling for :s (this is the fix for issue #42). Newlines in the command will properly be matched. We also properly display the affected lines and the number of matches when the /n flag is provided.
• Adds support for the /m regex flag in substitutions. vim handles this situation differently, providing various ways to indicate you're willing to match newlines with . in the regex or whatever, but I'm trying not to deviate too far from more standard regex syntax if possible.
• Report the number of affected lines and the number of replacements even when the /n flag isn't provided. Before you had to add /n to the end of the pattern to see the lines/replacements, but when you ran the actual substitution that information was not reported. It is now reported when you run the regular substitution as well.
• Fix an issue where the new status view would not display messages if any status components changed during the same run loop cycle as a message was displayed. This would occur if, for example, a message was displayed during a command that also updated the caret (like :substitute). We no longer hide messages when a status component changes if that change occurs during the same run loop cycle as the message was displayed.

[Shadowfiend]

I would highly appreciate it if folks would use this branch for a little bit this week and see if it interferes with any of your day-to-day :s usage. I will be doing the same :)

[Shadowfiend]

There is at least one issue with infinite looping going on in the new transform implementation, which can be triggered by trying to add a new function on a new line at the beginning of the line in an Objective-C header file. I'll be investigating this tonight or tomorrow.

[Shadowfiend]

Looks like this gentleman isn't properly respecting replacement ranges (it always operates on the whole file). Fix to come.

[Shadowfiend]

Finally fixed the range issues with this branch. Going to use it for a couple of days and see if everything seems kosher; if it does, then I'm going to go ahead and merge the branch unless I hear something :)

[Shadowfiend]

Merging this lovely lady in; please file an issue if you run into any problems with the new implementation.