add `replace` option

I added `replace` option to replace tail64n substring in a string on expanding. 

This is to satisfy an usecase like below:

1. Assume logs are generated by multilog (daemontools)
2. Use fluent-agent-lite or in_tail (fluentd) with format none option. They should send logs like

    {"message":"@4000000052f88ea32489532c [FATAL] blah blah"}

3. Expand tai64n formatted time in the messages just like `tai64nlocal` using fluent-plugin-tai64n_parser. tail64n substring is replaced with the human-readable time string. 

    {"message":"2014-02-10 17:32:25.612979500 [FATAL] blah blah"}

4. Store to mongodb.

Notice that in this usecase, I do not use fluent-plugin-parser. So, `replace` option is required.