music21.text.assembleLyrics: support custom word separator

[oxygen-dioxide]

This PR adds custom word separator support for music21.text.assembleLyrics.

Unlike European languages, in East Asian languages, words are not separated with space.

For example, This is a song's lyric extracted by music21:

也 许 很 远 或 是 昨 天 在 这 里 或 在 对 岸 长 路 辗 转 离 合 悲 欢 人 聚 又 人 散 放 过 对 错 才 知 答 案 活 着 的 勇 敢 没 有 神 的 光 环 你 我 生 而 平 凡

After this PR, I'll be able to extract the lyrics like this with assembleLyrics(stream, wordSeparator='')

也许很远或是昨天在这里或在对岸长路辗转离合悲欢人聚又人散放过对错才知答案活着的勇敢没有神的光环你我生而平凡

#1787 will be fixed.

[coveralls]

coverage: 92.993% (+0.003%) from 92.99%
when pulling 3224b25 on oxygen-dioxide:custom-word-separator
into 3516adf on cuthbertLab:master.

[coveralls]

coverage: 92.992% (+0.002%) from 92.99%
when pulling 7b8f4af on oxygen-dioxide:custom-word-separator
into 3516adf on cuthbertLab:master.

[oxygen-dioxide]

Also I would like to add few Chinese folk music, such as Mo Li Hua into our corpus, for developers who want to test functionalities on East Asian languages. What do you think?

[mscuthbert]

Thanks! I'm trying to make less-used additions keyword only and thought that all your tests had this. Will merge as soon as things pass.