Support env file

[umputun]

relatated to #126

This PR adds basic support for env file in yaml format. The default is env.yaml but can be changed in the command line. In case this file is missing, it won't fail and will continue as usual, so the change is back-compatible.

example of env.yml:

vars:
  var1: val1
  var2: val2

[umputun]

note: the top level vars added intentionally as some day we may want to extend the format to more sophisticated use case

[coveralls]

Pull Request Test Coverage Report for Build 5501284659

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 21 of 25 (84.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 84.683%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/spot/main.go	21	25	84.0%

Totals
Change from base Build 5432095431:	-0.01%
Covered Lines:	2488
Relevant Lines:	2938

---

💛 - Coveralls