Let children() method use optional filter argument

As mentioned in GitHub #38 issue, let children() method
allow optional filter argument as `qr//` or code reference.
If invalid argument is given then children() will be croaked.
I setted code reference received two arguments, the first one
is `$self` and the second one is the name of the child to avoid
some object creation/stringification overhead.  I can't convince
`$self` is really needed or not, but it was always useful in my cases.
Added new tests for various argument of children()

- Regexp argument
- code reference argument
- invalid argument
Use `$/` rathern than `\n` for portability
Child name shoud be put into localized `$_` for coderefs
restrict children() to only take Regex argument
v0.028

    [ADDED]

    - The 'children()' method now takes an optional regular expression to
      filter the results
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