Add tf 0 12 support

[eversC]

Adds support for terraform v0.12

On the face of it this will look like a tonne of changes so I'll make an attempt at summarising:

• refactored most regex processing logic out of the main() func into various dedicated functions. I tried to keep as much code as I could the same when copying over.
• added struct called match which holds all of the regex group matches from retfplanline. This wasn't exactly necessary but felt cleaner holding it all in its own struct
• added an expression struct, which holds regexes that differ between versions of terraform.
• added a versionedExpressions map which maps terraform version to an expression
• added a switch via env var TFENV so users can use tf 0.12 (defaults to tf 0.11)
• regexes for tf 0.12 (was planning on trying to make the original regexes applicable to both versions but it became too cumbersome)
• unit tests for both tf 0.11 & 0.12 in main_tests.go

In terms of testing, I've done a diff (see attached) of the outputs from running make test with tf 0.11 in the /tests directory, on the latest binary you've released Vs. a binary I've built from my branch. I think it's come out well; just a few differences where the id of the random_string.some_password is different (which is fine).
tf-011-old-vs-new-diff.txt

For what it's worth, I've also run make test with tf 0.12 (see attached output). This looks okay to me but might need some sanity checking.
tf012_test.txt

[osterman]

@eversC this looks amazing! Thanks so much for biting the bullet and helping us upgrade this.

[osterman]

@eversC this part looks suspect: should id there be masked?

[osterman]

fixes #6

[eversC]

Hi @osterman 👋 , thanks for getting back to me so quickly.

@eversC this part looks suspect: should id there be masked?

That looks like a bit of an oversight on my part, as it does seem id should be masked. It looks like tfmask currently exhibits that behaviour too, though, unless I'm missing something?

[eversC]

I made an additional commit, 2633d0e, which I believe fixes the random_string.some_password.id exposure issue.

I just updated the tfmaskValuesRegex and tfmaskResourceRegex regexes to include "id" and "random_string" respectively.

[osterman]

It looks like tfmask currently exhibits that behaviour too, though, unless I'm missing something?

Yea, not improbable =)

[osterman]

LGTM! Changed default version of terraform to 0.12. Will merge and cut a release. Thanks for this!