Add option for removing the leading path element. 

Adds `:strip-leading-path` to the `:tar` options hashmap.
If `:strip-leading-path` is set to true, then the leading directory path
element (in this plugin's case the project name) will be stripped.

This gives the same result as running the folowing in the command line:
`tar czf lein-tar-3.2.0.tgz -C lein-tar-3.2.0/ .`

Example tar archive contents for values of `:strip-leading-path`:

 false (default) | true
-----------|---------------------
 lein-tar-3.2.0/                    | 
 lein-tar-3.2.0/bin               | bin/
 lein-tar-3.2.0/bin/heyo      | bin/heyo
 lein-tar-3.2.0/etc               |  etc/
 lein-tar-3.2.0/etc/foo.conf | etc/foo.conf
 lein-tar-3.2.0/lib/               | lib/
 lein-tar-3.2.0/lib/lein-tar-3.2.0.jar | lib/lein-tar-3.2.0.jar