Add terraform checkers

[marsam]

Hi, I've added checkers for terraform

Tests:

make SELECTOR='(tag checker-terraform)' integ

make SELECTOR='(tag checker-terraform)' integ
cask exec emacs -Q --batch -L .  -l maint/flycheck-compile.el -f flycheck/batch-byte-compile flycheck.el
cask exec emacs -Q --batch -L .  -l maint/flycheck-compile.el -f flycheck/batch-byte-compile flycheck-ert.el
cask exec emacs -Q --batch -L .  --load test/flycheck-test --load test/run.el -f flycheck-run-tests-main '(and (tag external-tool) (tag checker-terraform))'
Loading /Users/marsam/code/flycheck/.cask/26.2/elpa/julia-mode-20190407.2119/julia-latexsubs...
Running tests on Emacs 26.2, built at 2019-06-18
Running 1 tests (2019-06-18 22:01:19-0500)
   passed  1/1  flycheck-define-checker/terraform/default

Ran 1 tests, 1 results as expected (2019-06-18 22:01:20-0500)

make SELECTOR='(tag checker-terraform-tflint)' integ

make SELECTOR='(tag checker-terraform-tflint)' integ
cask exec emacs -Q --batch -L .  -l maint/flycheck-compile.el -f flycheck/batch-byte-compile flycheck.el
cask exec emacs -Q --batch -L .  -l maint/flycheck-compile.el -f flycheck/batch-byte-compile flycheck-ert.el
cask exec emacs -Q --batch -L .  --load test/flycheck-test --load test/run.el -f flycheck-run-tests-main '(and (tag external-tool) (tag checker-terraform-tflint))'
Loading /Users/marsam/code/flycheck/.cask/26.2/elpa/julia-mode-20190407.2119/julia-latexsubs...
Running tests on Emacs 26.2, built at 2019-06-18
Running 1 tests (2019-06-18 22:02:06-0500)
   passed  1/1  flycheck-define-checker/terraform-tflint/default

Ran 1 tests, 1 results as expected (2019-06-18 22:02:08-0500)

[fmdkdd]

LGTM. Thanks!

@cpitclaudel ?

[cpitclaudel]

Superb :)