normalize_path: Add Append mode

[daalfox]

Closes #530

[daalfox]

I tried to just solve the #530 in this PR. But I think we can improve this further. Currently merging slashes are happening implicitly which I think should be explicit. I can also refactor this and split the code for different modes.

[daalfox]

As for the naming, I added a new function that takes mode but maybe we want to add some convenience functions. I suggest normalize_trim and normalize_append or just trim and append because we know it's an http specific service already. Maybe merge_slashes, merge_trim, merge_append in future?

[daalfox]

@jplatte Can you check this out? I understand you might be busy, just tagging in case you missed it.

[jplatte]

I saw, just busy. Feel free to ping me again in a week or two if I haven't responded then.