Add -S option to auto-derive session name from pwd

[vi]

I got tired of starting enki as

project1$ enki -s project1
myotherproject$ enki -s myotherproject

and implemented option to automate this.

Currently it is based on v15.11.1 (master fails for me now: something about Qt5 vs Qt4).

Are you interested in the feature (so I try to prepare it for master) or it shall be another dangling patch for rebasing?

[andreikop]

Cool idea!

Can we use the same -s (small) flag but autodetect session if argument is empty?

[vi]

-s is already busy for explicit session name on command line.

Making semi-flag semi-argumentful command line options is a bad design: enki -s Makefile main.c - initialize session per current directory with two files or initialize session "Makefile" with one file?

[andreikop]

OK, open a pull request which works with the current master

[vi]

Rebased the pull request to master.

[andreikop]

Perfect!