Add a way to assign a variable "in-line"

Rather than using the shellAssign macro, it'd be nice to be able to assign the result of command to a variable "in-line". To do so we could use a special token such as "=>" followed by the variable name, = and the command to execute. For example:

```nim
var firstResult = ""
var secondResult = ""
shell:
    =>firstResult = cat myfile.txt
    =>secondResult = ls
```
