Remove trailing slashes from filename directory argument

[verhovsky]

Description

I did this:

mdl source/docs/

Environment

macos

MDL Version
0.13.0

Expected Behavior

source/docs/awk.html.markdown:6: MD032 Lists should be surrounded by blank lines
source/docs/awk.html.markdown:6: MD034 Bare URL used
source/docs/ballerina.html.markdown:427: MD002 First header should be a top level header
source/docs/ballerina.html.markdown:8: MD009 Trailing spaces
source/docs/ballerina.html.markdown:13: MD009 Trailing spaces
...

Actual Behavior

source/docs//awk.html.markdown:6: MD032 Lists should be surrounded by blank lines
source/docs//awk.html.markdown:6: MD034 Bare URL used
source/docs//ballerina.html.markdown:427: MD002 First header should be a top level header
source/docs//ballerina.html.markdown:8: MD009 Trailing spaces
source/docs//ballerina.html.markdown:13: MD009 Trailing spaces
...

there are two slashes