Customizable settings for skipping questions

Creating a new session in a simple project asks many questions for which
the default answers may be absolute fine.  This change adds four
customizable variables, and setting them (in file-local and dir-local
variables) would allow questions to be skipped during session creation.

See #407 

For example:

```
-- Local Variables:
-- haskell-session-ask-new-project: nil
-- haskell-cabal-use-directory: t
-- haskell-session-current-directory: t
-- haskell-session-use-default-target: t
-- End:
```