Allow for custom testcases with the leetcode test command, and some minor edits

[Raees678]

Now allows for an optional command line argument after leetcode test so that a custom test case can be provided. Test cases provided can be multiline with a \n.

Adds to the .gitignore .DS_Store and .idea files. Now macOS trash files and CLion/Intellij IDE support files are ignored.

Also makes a slight visual change so that when test cases are displayed in the CLI a ↩ (newline) character is used instead of a , (comma) to signify multi line input test cases.

[clearloop]

Thanks!!! I've already wanted to add test cases but forgot about it recently...

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btw, Have you ever seen a lot of

warning: use of deprecated item 'std::error::Error::description': use the Display impl or to_string()

in the current version?

My rustc version is rustc 1.43.0-nightly (61d9231ff 2020-02-15), and I'm not sure what will happen if I fix these warnings in my environment XD.

I'll merge this pr now, and will do some fix jobs on these warnings, then we can publish a new version as v0.2.15!

[Raees678]

I see those warnings too (rustc 1.41.0 (5e1a79984 2020-01-27), but didn't really want to try and investigate too much for fear of breaking the existing project. Thanks for the merge, this project has really streamlined my leetcode grinding so its nice to contribute to something I actually use!

[clearloop]

I'm really touched by your pr, bro!

Your pr is very meaningful to me! This is the first time that somebody else pull request to my project in my life, and it also means that I did write a project can be used by others!!! Anyway, I'll do whatever I can to improve this project!