shelve: rename nodestoprune to nodestoremove As per feedback from the community.

templater: port pad() to take keyword arguments This is another example where keyword arguments can be actually useful.

templater: add support for keyword arguments Unlike revset, function arguments are pre-processed in templater. That's why we need to define argspec per function. An argspec field looks somewhat redundant in @templatefunc definition as a name field contains human-readable list of arguments. I'll make function doc be built from argspec later. Ported separate() function as an example.

templater: add parsing rule for key-value pair Based on the revset implementation, 70a2082f855a. This patch also adjusts the test as '=' is now a valid token.