Generalize regexps to match left and right timestamps

I modified `erc-view-log-get-keywords` a lot, so I don't know whether
you will accept this pull request.  If you want me to make some
improvements/modifications, I am ready.

Here is an example of erc log for testing `erc-view-log-mode`:

    [Wed Jan 22 2014]
    *** Users on #bots: alezost Grace HelenaKitty @ChanServ seedbot lapilofu
        duckie  [00:01]
    *** #bots modes: +cnt
    *** #bots was created on 2009-12-15 16:07:15
    <Grace> Hello alezost, Each one of us is born with the innate ability to
            resolve whatever difficulties we face.
    <alezost> Grace: tell me something about anything
    <Grace> I don't know what anything is
    <alezost> Grace: shame on you then  [00:02]
    <Grace> On me?
    * alezost is setting `erc-insert-timestamp-function' to
      `erc-insert-timestamp-left'  [00:06]
    [00:06]<alezost> seedbot: hi mate
           <Grace> Howdy alezost
    [00:07]<alezost> Grace: go away
           * duckie snuggles up to alezost.
           <Grace> Go away? Where is that? Someplace you have been or are going
                   back to? ;)
           ERC> /wi Grace 
           *** Grace is Grace (~Grace@c-50-134-7-163.hsd1.va.comcast.net)
           *** Grace is on channel(s): #bots 
           *** Grace is/was on server sendak.freenode.net (Vilnius, Lithuania, EU)
           *** Grace has been idle for 00:00.10, on since 2014-01-20 00:09:29
           <alezost> thanks bots, it's the best channel
    [00:08]*** You have left channel #bots