Fix `haskell-interactive-switch` to match `switch-to-haskell`

This switches to the REPL even if the REPL window is already visible
and should thus fix #155.
Remove `haskell-{hugs,ghci}` including most references

Those bitrotting modules have been deprecated for quite some time now,
and haskell-mode.el has been informing users since ce945dc (i.e. 5+
years) about their deprecation. This closes #170.
Update README w.r.t. supported Emacs versions
Extend regexps in interactive-mode to accept `-ferror-spans`

This just generalizes the regexps used for matching error lines in
`haskell-{process,interactive-mode}.el` to allow for an optional 2nd
column number (which then forms a span "startcol-endcol")

This is a step for addressing #151