search: Adjust log levels for max search depth

When a user has specified a specific max search depth (--depth), it is
reasonable to assume that they are aware of it. In this case, it should
suffice to output this information on the debug level.

Relatedly, it is not (in my opinion) an error to exceed the max search
depth, since the program is designed that way. I've lowered it to a
warning.

This increases, in my experience, the user friendliness of the --depth
option, since it outputs a lot less noise.
Fix win32 build
Auto enable color when CMDER is running
Merge pull request #500 from larsch/detect-cmder

Auto enable color when CMDER is running
Merge pull request #398 from catharsis/search-depth-noise

search: Adjust log levels for max search depth
Merge pull request #495 from mattn/win32fix

Fix win32 build
Remove unnecessary goto
Completely ignore stdin when searching given path

Piping into ag used to turn off the pretty output format, even if files
provided via command line argument were searched.
Allow stdin redirection

If `ag foo <file` is used, try to search the file like a file and not
like a stream. This has better performance and supports multiline
regexes. Fall back to stream behaviour if the system doesn't support
`/dev/stdin` or `/dev/fd/0` as this is still better than not supporting
it at all.