Allow set tool used as shell

[mvoidex]

This is very useful for me to set PowerShell as tool to run commands with.

There are new setting shell which is used to define command. This setting can contain $cmd variable, which is replaced with command after all decorations (adding prefix etc.). If shell is string, then after replacing $cmd it will be used as shell command. If shell is list, then it is interpreted as program with arguments, where arguments can contain $cmd and one of arguments can be $input (in this case input will be passed as arg, not via stdin). I've added $stdin because I failed to invoke powershell reading stdin.

Another feature is vars allowed in command input:

• $file will be replaced with view file name
• $path with full file name
• $dir with view file directory

Here's my settings i'm using with this branch:

{
    "shell": ["powershell", "-Command", "& { $cmd }", "$input"]
}

[ci70]

Please implement this in the main brach. Thanks.