Vscode-restclient: Add support for default headers

[Kronuz]

• VSCode Version: 1.22.2
• OS Version: macOS High Sierra 10.13.4
• REST Client Version: 0.18.3

Please add rest-client.defaultHost, rest-client.defaultContentType, rest-client.defaultAccept, etc.; so that, if configured, we don't have to specify http://localhost:8080/ or Host: localhost:8080.

The idea is that for things like:

PUT /documents/john
Host: localhost:8080
Content-Type: application/json
Accept: application/json; indent=4

{
    "firstname": "John",
    "lastName": "Smith"
}

One could simply configure the (workspace) settings:

{
    "rest-client.defaultHost": "localhost:8080",
    "rest-client.defaultContentType": "application/json",
    "rest-client.defaultAccept": "application/json; indent=4"
}

and use:

PUT /documents/john

{
    "firstname": "John",
    "lastName": "Smith"
}

The proposal is to convert settings of the form PascalCase to dash-separated (defaultHeaderName to header-name); so this would effectively extend rest-client.defaultuseragent (and rename it to rest-client.defaultUserAgent).

Edit:
I'd propose support for the following standard headers:

• Accept
• Accept-Charset
• Accept-Datetime
• Accept-Encoding
• Accept-Language
• Access-Control-Request-Headers
• Access-Control-Request-Method
• Authorization
• Cache-Control
• Content-Type
• Cookie
• Host
• Origin
• User-Agent

[Huachao]

@Kronuz I have added some comment in #206, and what about

{
    "rest-client.defaultHeaders": {
         "X-Custom-Header": "foo bar",
         "Host": "localhost"
    }
}