Add id to bsCollapsePanel

[warnes]

• Adds optional id tag to bsColllapsePanel
• Adds bsCollapseButton and bsCollapseLink functions to generate buttons/links that toggle a bsCollapsePanel.
• Update Collapse example to use these.

Resolves #132

(Also adds renv environment for development convenience. I'll be glad to drop it if you prefer.)

[warnes]

To maximize backwards compatibility, the current parameter list for bsCollapsePanel and bsCollapseLink is:

function(title, ..., id, value = title, style = NULL)

with id being optional.

It would better match the usual shiny pattern to put id first, and make both id and title required, thus:

function(id, title, ..., value=id, style=NULL)