Fixed line joining

(1) If more than one line (lets say n) was selected in linewise selection, then n+1 lines were joined. Now we join only n lines.

(2) If the cursor was on line containing only newline and we joined it with another
line, then those lines were glued with space. Experiments in Vim show that the
space is not added in this case (although this behavior is not documented in manual), therefore we are omitting it now. I added a unit test for this case.

(3) When count was specified, it joined count+1 lines. According to the Vim manual, exactly count number of lines must be joined with minimal number of 2 lines.
I also changed two unit tests to consider this behavior.