Document and test using SECRETS_DIR environment variable

[joshrabinowitz]

For #247 , which supports changing the name of the .gitsecret directory via the SECRETS_DIR env var.

@lrascao said:

Regarding the tests and since this is a global option that affects all commands, what would be a good strategy?

[joshrabinowitz]

Some ideas on how to test this feature:

Automate the test run to run every test twice, once with SECRETS_DIR set, and once without.

• Pros: Comprehensive.
• Cons: Doubles testing time.

OR

Always set the SECRETS_DIR env var in the test setup code and assume that the default .gitsecret works. (Edit: this was implemented in #251)

• Pros: Simple to implement. This is probably the right thing to do first.
• Cons: We're assuming it works without setting SECRETS_DIR.

OR

Tweak to above: Do the above test, and add a test for each of the test files that specifically doesn't set the SECRETS_DIR env var.

• Pros: Comprehensive, probably the right way to go in the long run.
• Cons: Sort of a lot of code to write.

[joshrabinowitz]

#251 adds the test "Always set the SECRETS_DIR env var in the test setup code and assume that the default .gitsecret works" described above. It also documents use of the SECRETS_DIR env var.

git-secret.io should reflect this feature when the next version is released.
Above moved to #250

[joshrabinowitz]

Since git-secret.io doc item moved to #250, this is complete