Add show argument to bs_append.bsplus_accordion()

[be-marc]

Hey,

I added an argument to bs_append.bsplus_accordion(), which suppresses that the first panel of an accordion is always open. I need this for my https://github.com/be-marc/swdt project.

[ijlyttle]

Sorry for the delay on this - this would fix #35.

I plan to work with this and merge it once I have worked on the documentation (so I can re-learn everything).