feature: support set extension for dump file

[almas1992]

fix #4

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[doujiang24]

@almas1992 Thanks for your contribution, please fix the code style issue, refer: https://github.com/mosn/holmes/pull/130/files#diff-f0067949ae0f48685cd310e00ccaf0c72d02048ccc6aeb0a862cd30b81e5e566

Also, please add doc in README, thanks.

[almas1992]

@almas1992 Thanks for your contribution, please fix the code style issue, refer: https://github.com/mosn/holmes/pull/130/files#diff-f0067949ae0f48685cd310e00ccaf0c72d02048ccc6aeb0a862cd30b81e5e566

Also, please add doc in README, thanks.

These code style issue you mentioned are not brought about by my PR，I just add one value defaultDumpFileExtension and it used in my code .

不知道是我没表达清楚还是，你所说的代码风格问题，即有两个未使用变量不是我这次提交引入的，所以我也不太好直接去除，还希望作者自行定夺

[almas1992]

Removed unused variables to fixed code style issue and upgrade github.com/shirou/gopsutil version to fix TARGET_OS_MAC' is not defined, evaluates to 0 [-Wundef-prefix=TARGET_OS_] issue

[almas1992]

just close it.