Support a directory as input, with subdirectories

**Is your feature request related to a problem? Please describe.**

The problem is that it's not possible to set a entire directory in INPUT, and get all the logs in the directory including subdirectories.

Using the `tail` plugin, I can set:
```
[INPUT]
Name tail
Path /var/log/*
```
But it will only get the files immediately under `/var/log`

**Describe the solution you'd like**

The way to achieve this in FluentD is to use `**`, which matches recursively subdirectories. We can have the same solution here.

**Describe alternatives you've considered**

Maybe a flag `recursive=True`, but I guess it's less intuitive and harder to code.

**Additional context**