pull/export: allow exporting to same directory+level

[odeke-em]

Fixes #660.

Allow exporting to same directory and level, instead of:
a) Always dumping exports per exportable file in a sub-directory
called <filename>_exports.
This CL allows for users to request that their exports be put in the
same directory with pull flag --same-exports-dir.
If --same-exports-dir is set, it will place all exports in the same
directory and level.

• Exhibit A:

 drive pull --explicitly-export --exports-dir ~/Desktop/exp --export
pdf,txt,odt --same-exports-dir
Resolving...
+ /test-exports/few.docs
+ /test-exports/few
+ /test-exports/influx
Addition count 3
Proceed with the changes? [Y/n]:y
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/influx' to '/Users/emmanuelodeke/Desktop/exp/influx.pdf'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/influx' to '/Users/emmanuelodeke/Desktop/exp/influx.txt'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few' to '/Users/emmanuelodeke/Desktop/exp/few.pdf'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few.docs' to '/Users/emmanuelodeke/Desktop/exp/few.docs.txt'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few.docs' to '/Users/emmanuelodeke/Desktop/exp/few.docs.odt'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few.docs' to '/Users/emmanuelodeke/Desktop/exp/few.docs.pdf'

• Exhibit B (Original case, _exports suffixed dirs used):

$ drive pull --explicitly-export --exports-dir ~/Desktop/exp --export
pdf,txt,odt
Resolving...
+ /test-exports/few.docs
+ /test-exports/few
+ /test-exports/influx
Addition count 3
Proceed with the changes? [Y/n]:y
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/influx' to '/Users/emmanuelodeke/Desktop/exp/influx_exports/influx.pdf'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/influx' to '/Users/emmanuelodeke/Desktop/exp/influx_exports/influx.txt'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few' to '/Users/emmanuelodeke/Desktop/exp/few_exports/few.pdf'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few.docs' to '/Users/emmanuelodeke/Desktop/exp/few.docs_exports/few.docs.odt'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few.docs' to '/Users/emmanuelodeke/Desktop/exp/few.docs_exports/few.docs.txt'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-exports/few.docs' to '/Users/emmanuelodeke/Desktop/exp/few.docs_exports/few.docs.pdf'