[Enhancement] Add configure detection

[heyanlong]

Add configure detection, if it does not exist, automatically skip loading.
If open ~/.skywalking.yml no such file, output warning and skip the file

[kezhenxu94]

I’m not sure whether we should log warning messages when the config file is absent, because it’s completely optional and is just for reusing options in command line, when the users want to just run one or two commands for a quick look with all options given in command line, the warning message breaks the output json format and forces the users to create a configuration file there, WDYT @wu-sheng

[hanahmily]

We can send these warning logs to STDERR.If the user wants to ignore them, issue swcli ... 2>/dev/null to throw them to trash.

[wu-sheng]

I think log into a file is not suitable for CLI. Recommend debug log, whicb we have an optional to open it.

[wu-sheng]

This is a not issue for CLI, just helps for debuging cli itself or user env, right?

[codecov-io]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   64.28%   64.28%           
=======================================
  Files           5        5           
  Lines          84       84           
=======================================
  Hits           54       54           
  Misses         24       24           
  Partials        6        6

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[kezhenxu94]

Merging