Default variable values

[kyleect]

• Default to using default env if one isn't passed during templating
• Default to using default when resolving variable values on a parsed request file
• Allow envs to skill defining a value for a variable if a value was defined in the default environment

```%config
vars = ["greeting", "name"]

[envs.default]
greeting = "Hello"

[envs.dev]
greeting = "Greetings"
name = "Tester"

[envs.prod]
# greeting = "Hello"
name = "World"
```

```%request
POST https://httpbin.org/post HTTP/1.1
content-type: application/x-www-form-urlencoded

say={{:greeting}}&to={{:name}}
```

```%response
HTTP/1.1 200 OK
```

Scenarios

Single Variable

Default environment returns default env value

Given variables ["value"]
And envs.default.value = 123
And envs.test.value = 456
When env is "default"
Then the variable value is 123

Defined environment returns defined env value

Given variables ["value"]
And envs.default.value = 123
And envs.test.value = 456
When env is "test"
Then the variable value is 456

Multi Variable

Given variables ["valueA", "valueB"]
And envs.default.valueA = 123
And envs.test.valueA = 456
And envs.test.valueB = 999
When env is "default"
Then there is a validation error that valueB isn't resolved

Given variables ["valueA", "valueB"]
And envs.default.valueA = 123
And envs.test.valueA = 456
And envs.test.valueB = 999
When env is "test"
Then the variable valueA is 456
And the variable valueB is 999