TS-2364: Introduce slice notation to field syntax in log format

Now, we have two kind of field syntax in log format:
(1) simple syntax: '%<field>', for example: %<cqu>
(2) container field, syntax: '%<{field}container>',
    for example: %<{Referer}cqh>.

This patch introduces slice notation to field syntax, so that we can easy to
limit the length of filed's output. With slice notation, the filed syntax will
looks like:
(1) '%<field[start:end]>'
(2) '%<{field}container[start:end]>'

In both forms, slice notation can be omitted, which means the whole field.

But with one limitation: slice notation makes sense only when the field is
string type and it shouldn't be ip/timestamp which are converted to
string from integer.

We can see the syntax of slice notation from Python, Golang, it's pretty
simple:
  [start:end] //items start through end-1
  [start:]    //items start through the rest of the array
  [:end]      //items from the beginning through end-1
  [:]         //the whole array(by default)

For example,
  '%<cqup>'       //the whole characters of <cqup>.
  '%<cqup>[:]'    //the whole characters of <cqup>.
  '%<cqup[0:30]>' //the first 30 characters of <cqup>.
  '%<cqup[-10:]>' //the last 10 characters of <cqup>.
  '%<cqup[:-5]>'  //everything except the last 5 characters of <cqup>.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>