Outdent only if a tab is matched at the start of the line

Fixes https://github.com/atom/atom/issues/2560. 

When outdenting, the `leadingTabRegex` currently matches spaces only at the beginning of the line, while tabs are matched anywhere in the line. As a result, the outdent method would eat up legit characters on lines which contain tabs after the beginning of the line.

This change tweaks the regex so that tabs are matched only at the beginning of the string.

cc @probablycorey @kevinsawicki