[#42] Specifying config via environment variable

[DK318]

Description

Problem

We can't specify config file via environment variable. Suppose we don't like default config.toml file and want to use different config file. And writing --config my-lovely-config.toml in each command looks very odd.

Solution

Supported specifying config file via COFFER_CONFIG environment variable. --config has presedence over this variable.

Related issue(s)

• Fixes #42.

  Related changes (conditional)

  • Tests

    • [ ] If I added new functionality, I added tests covering it.
    • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

  • Documentation

    • [x] I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock

  • Public contracts

    • [x] Any modifications of public contracts comply with the Evolution of Public Contracts policy.
    • [x] I added an entry to the changelog if my changes are visible to the users and
    • [x] provided a migration guide for breaking changes if possible

  Stylistic guide (mandatory)

  • [x] My commits comply with the policy used in Serokell.
  • [x] My code complies with the style guide.

[MagicRB]

LGTM

[dcastro]

@MagicRB for future reference, please go through "Files changed > review changes > Approve" to approve a PR