introduce Order struct to consistent order of result.

[tamada]

#30

The cause of this bug is to open too many files.
In the current implementation, wildcat opens files at once and collects all files, then counts collected entries in each goroutine.
Therefore, too many entries may cause the hang by the limit of the file handers.

To fix the bug, I will update as follows.

• introduce Order struct for consistent orders in the results.
• no collect entries, and count each entry in each goroutine.
• collect Result for printing results.