src/anki/ankiclient: add {pitch-categories} marker

[precondition]

Add a new Anki marker called {pitch-categories}. This marker is a comma-separated list of all the distinct pitch accent categories applicable to the term. For example, the {pitch-categories} of 赤い would be kifuku,heiban (or heiban,kifuku — no guarantees can be given in regards to the order).

All the available pitch accent categories are:

1. heiban (pitch position = 0)
2. atamadaka (pitch position = 1)
3. nakadaka (1 < pitch position < term.moras.size)
4. kifuku (1 <= pitch position < term.moras.size ; only applicable to i-adjectives and non-suru verbs)
5. odaka (pitch position = term.moras.size)

The corresponding marker in Yomitan is actually called {pitch-accent-categories} but given that Memento omits the -accent- part in its versions of all the other pitch accent markers, I stuck to Memento's naming style.

Partially solves #250.

[ripose-jp]

There's some trailing white space in some of the files you changed. You can remove it by running find . -type f -print0 | xargs -0 perl -pi -e 's/ +$//' in the src directory. There's probably a setting in your text editor that will remove it for you in the future.

Also, could you amend your commit message to include some of the info you put in your PR description?