Fix autocomplete for environment/global variables

[halvarsson]

This PR fixes #55, which impacts the autocomplete of environment / global variables, as colon is not considered a word separator, but this is valid for PowerShell variables

It should now autocomplete correctly

[mklement0]

Thank your for the speedy fixes.

#55 described two additional symptoms, both of which I now realize can be considered feature requests:

• Suggestion: make Enter an alternative keypress to Tab when selecting a completion from a pop-up list #59

• Suggestion: Don't insert a literal Tab character when tab-completion finds no possible completions #60