beforeInput: Rename "inputCharacter" to "inputText"

[yosinch]

One user action can insert multiple characters instead of one character. "Character" suffix mislead web authors.

[chong-z]

Agree that using insertText is better since it's a more standard command name defined in https://developer.mozilla.org/en-US/docs/Web/API/Document/execCommand#Commands. (See discussion in #79 for the list of inputType to start with)

[johanneswilm]

Hey, @choniong , I couldn't merge your PR due to some restriction set in the W3C's github group, but I made the change nevertheless, as there didn't seem to be any opposition.