Feature request: add user-defined dictionary

[szsdk]

When I write some scientific stuff, I need to use some words which are not in the default dictionary or even non-English words. I know I can use the you_expand_me to make a workaround. But the expansion and the dictionary are still two different things. I think this function would be quite related to this issue --- #85 (comment) .

[dongyuwei]

Okay, it's doable. Another thing is the trie data structure is prebuilt && serialized in dictionary/google_227800_words.bin, we need a way to build the trie and sync it in runtime.

[dongyuwei]

Another important thing is how to set the frequencies of new words.