Allow TF as overriding env var for cmd

Also shorten cmdpath to cmd, since the executable name can be used instead of the full path now.

With updated Rust deps and clippy fixes

No feature changes, just minor dependency updates

Allow passthrough args and pass back error code

• Allow passthrough args to terraform destroy by using positional arguments (see README)
• Passes back error code from all command line executions, and return that as the error code for the program. For other error, the error code is 1.

Better unit tested binary

• Added a lot more unit tests with some code refactoring, to improve the quality of the binary.
• No behavioral changes to the binary.

No longer finding .tfzignore recursively

Includes #18 (issue: #14), which is a breaking change and no longer find .tfzignore file recursively from current working directory, all the way back to the root directory, to find the first .tfzignore file it manages to do so.

Instead it only accepts .tfzignore file, and if missing, the command essentially becomes terraform destroy.

As such, if any solution assumes that there is some shared .tfzignore file across the repository, this would begin to fail. Individual .tfzignore on each terraform directory must be provided (which is likely to be the better practice also).

Also includes improvements from #16 (ASCII print fix for Windows) and #13 (incorrect verbose print).

Add install script

• Start to use -linux-x86_64 as suffix for Linux releases

Update README with more details

• No changes to binary
• Fix previous issue with cargo publish

First working binary using `.tfzignore`

Add statically linked binary for GitHub Release (#3)

* Allow auto-publishing when there is new tag

* Bump version and change to tfzignore

* Fix travis build issue

* Fix missing .env-build file