Issue 199 exec(command) function

[ntwrick]

Here is my proposal for an exec(command) builtin function to solve issue #199.

Considering that implementing a builtin function does not require new lexical support and that exec(command) is similar to what linux C libs provide and the golang exec module supports, it seems to me that this model is reasonably expressive as well as easy to implement.

See tests/test-exec.abs as example of an abs script that marshalls args for a an interactive editor and then executes it with live stdIO. This works in both linux with nano and win10 with notepad. It also works for commands in linux that terminate with & which means we can launch a true daemon process in the background as well.

As a bonus, the $var command string interpolation code has been moved to the util module where it can be shared by regular commands as well as exec commands.

[ntwrick]

Redo against branch 1.3.x.