support two superscripts, improve chord position

-- adds support for two superscripts
e.g. C_7_/F_#
-- improves centering of the chord name in order
to use more space in case it exits the image right
boundary
replace linear bars with arcs
Merge pull request #1 from psifidotos/patches

support two superscripts, improve chord position
Merge pull request #2 from psifidotos/ellipticBar

replace linear bars with arcs