convert some string keywords to bool

[takumakanari]

This p-r adds new feature that converts some string keywords to boolean value.

embulk-json-path has similar feature, added at takumakanari/embulk-parser-json#1.

[hiroyuki-sato]

Sorry, I forgot to push submit review button.
I think it is better to add some strings like the below for compatibility CSV parser plugin behavior.

How do you think?

private static final List<String> BOOL_TRUE_STRINGS = ImmutableList.of("true", "1", "yes","y","t","on");

[hiroyuki-sato]

The CSV parser treat the following value as true.
So I think it is better to add a some keywords (ex On) for true value.

How do you think?

ImmutableSet.of(
        "true", "True", "TRUE",
        "yes", "Yes", "YES",
        "t", "T", "y", "Y",
        "on", "On", "ON",
        "1");

https://github.com/embulk/embulk/blob/master/embulk-standards/src/main/java/org/embulk/standards/CsvParserPlugin.java#L36-L41

[takumakanari]

• Keywords will be compared with case-ignored, so we don't need all case variations
  https://github.com/hiroyuki-sato/embulk-parser-jsonpath/pull/13/files#diff-2d0fb975204a675b52cc8ea532734971R90

• Adding more words (e.g. 'on', 'y') is good idea. I'll handle it.

[takumakanari]

BTW: Is it better to follow CSVParserPlugin to convert string keywords to bool? What do you think about this?

[takumakanari]

@hiroyuki-sato Thanks for your review. As we discussed on Twitter, I added more words to convert to bool.
Could you check it again?

[hiroyuki-sato]

Thank you!
LGTM 👍
I've merged!
(We discussed this topic on twitter) (Japanese text)

[takumakanari]

Thank you!