Pipeline should have an insert and/or a replace method

[dhdaines]

Because the language-specific trimmers are seriously defective at the moment (see #149) in both lunr.py and lunr.js (they don't include \w for instance), I need to be able to replace them. Unfortunately this is difficult to do robustly, because the API (which is inherited from lunr.js, so it's not your fault!) only has add, remove, before and after.

I would like to be able to do:

builder.pipeline.replace(
    builder.pipeline.registered_functions["lunr-multi-trimmer-fr"], trimmer.trimmer
)

But instead I have to do this:

builder.pipeline.remove(
    builder.pipeline.registered_functions["lunr-multi-trimmer-fr"]
)
builder.pipeline.before(
    builder.pipeline.registered_functions["stopWordFilter-fr"], trimmer.trimmer
)