Support for Terraform Beta installs

[warrensbox]

Support for Terraform Beta installs

[warrensbox]

This a useful feature, especially if a user wants to test out a terraform beta release version.
A couple of developers suggested that we pass a parameter to list all the version (including betas). The default, however, will only show the official release.

For example:-
The default tfswitch will display:

  ▸ 0.11.7
    0.9.8
    0.11.13
    0.11.12
↓   0.11.11

The optional arg tfswitch --list-all or tfswitch -l will display all versions including rc and betas:

  ▸  0.12.0-rc1
     0.12.0-beta2
     0.12.0-beta1
     0.12.0
     0.11.13
↓    0.11.11

I will merge your changes to master and incorporate my changes to allow optional parameter --list-all list all tf version.
I will release this new feature on Monday (May 20).

[warrensbox]

I will look at the code tonight

[warrensbox]

@devorbitus
Would you be able test the changes before I release it?

Steps:

1. git checkout develop (your changes are merged to develop)
2. go build -v -o tfswitch
3. ./tfswitch 0.12.0-rc1 then terraform -v to verify
4. ./tfswitch -l or ./tfswitch --list-all - verify if it list all the versions

[devorbitus]

@warrensbox Tested it as well as looked over the code and it looks good to me.

[devorbitus]

You may want to make some reference to the new feature on the documentation page

[warrensbox]

I have released this feature with #45

It should install any version except for:

• 0.12.0-alpha4
• 0.12.0-alpha3

That's because the file was named incorrectly on the hashicorp release page.

For example, the file for 0.12.0-alpha2 looks like below:

However the the file for 0.12.0-alpha4 looks like below:

Notice the difference in how the file is named. I am assuming this might be an error on Hashicorp and I will open an issue with them. So far I have only noticed the discrepancy in these 2 versions - 0.12.0-alpha4 and 0.12.0-alpha3.