Only assign position when unpaired count is 0

Previously the position would be assigned anytime a pair
was found which could cause the wrong bracket to be highlighted
when they were unpaired.

Closes #19
Reuse regexes
Support wrapping multiple selections in brackets

Closes #8
Bind ctrl-cmd-m to select text inside brackets

This new command, bracket-matcher:select-inside-brackets selects all
the text on the inside of the current bracket pairs.

Closes #17
Prepare 0.30.0 release
Add command to remove brackets