Allow to change disk log destination

[renyuneyun]

• Add a builder to construct DiskLogStrategy.
• Move the default construction of DiskLogStrategy from CsvFormatStrategy to DiskLogStrategy.Builder.
• Allow to customize log destination.

[jefryjacky]

Can you create builder for set directory and also the set the folder name so it will be more flexible. because sometime user only want to change the folder name only.

[renyuneyun]

@jefryjacky I don't quite understand your words...
What's the difference between "folder" and "directory" (except they are usually used by windows and unix respectively)?

You could pass a directory name like /sdcard/MY/AWESOME/DIRECTORY to the method and that won't run into a problem.

[jefryjacky]

I mean the "logger"
directory = diskPath + File.separatorChar + "logger";

the "logger" is hardcoded, I suggest to be able to custom that name.

[renyuneyun]

@jefryjacky
I think you have misunderstood the meaning. The hardcoded "logger" string is only used in if (directory == null) which means the user (of the library) doesn't provide a customized directory to use.

If changing the subdirectory instead of "logger" is the only thing the coder wants to do, why doesn't him/her pass Environment.getExternalStorageDirectory().getAbsolutePath() + File.separatorChar + "THE_DIRECTORY" to the builder?

[orhanobut]

Sorry for the late response and thanks for the pull request. I wasn't able to review anything this time period. Apart from a few things, all looks good to me. Would you mind to make the following changes? Then we can merge and release a version for this.

• Missing tests, that'd be great if you add some tests
• Support annotation for nullability. Recently I merged a PR which introduced these annotations, which makes easier for kotlin.
• JavaDoc to cover public API. That helps a lot, especially for generic parameters such as path etc.
• Update README.md to reflect these changes.

[renyuneyun]

I have changed some, but with one thing not sure:
How shoud I test the builder? Just confirm the resulting DiskLogStrategy will log?