Fix an issue where multiple handlers weren't registering correctly

[blampe]

The use case here is this:

@listen_to("(.*)")
def do_some_fallback(msg):
    ...

@listen_to("(.*)")
def do_some_other_fallback(msg):
    ...

We want >1 function attached to one regex "(.*)". However due to the way Python compiles regexs, these handlers were overwriting themselves here:

PluginsManager.commands['respond_to'][re.compile(regexp, flags)] = func

To convince yourself what's happening:

$ python
Python 2.7.13 (default, Dec 18 2016, 07:03:34)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> mapping = {}
>>> mapping[re.compile('(.*)', 0)] = 1
>>> mapping[re.compile('(.*)', 0)] = 2
>>> mapping[re.compile('(.*)', 0)] = 3
>>> mapping
{<_sre.SRE_Pattern object at 0x10f418580>: 3}

[blampe]

cc @gotlium

[gotlium]

Hi, we need to test with our existing code. Or you can add some settings to enable this feature. Because we have a backward compatibility with slackbot.