Create merge_index function

[fletchapin]

Most of the logic required for component-level documentation is done, but right now you have to hardcode different index files for each possible combination of components. Instead, I'd like to write a function which intelligently combines indexes so you'd only have to create one index per component (e.g. one for floc, one for sed) and then combine them automatically (currently you'd have to create one for floc, one for sed, one for sed and floc combined, etc).

[fletchapin]

We will need the same functionality for appendices. We'll want to only include relevant sections of the global appendix