templater: add separate() template function A pretty common pattern in templates is adding conditional separators like so: {node}{if(bookmarks, " {bookmarks}")}{if(tags, " {tags}")} With this patch, the above can be simplified to: {separate(" ", node, bookmarks, tags)} The function is similar to the already existing join(), but with a few differences: * separate() skips empty arguments * join() expects a single list argument, while separate() expects each item as a separate argument * separate() takes the separator first in order to allow a variable number of arguments after it