Option to shift the timestamp of each comment to match the video

[Mr-Quin]

Sometimes the timestamp of video being played may not match exactly with the timestamp of the comments. For example a video release may have a longer intro than the one where the comments are sourced from, resulting in each comment showing up earlier than they should.
An option to fast-forward or delay the comments should be added.

[Mr-Quin]

Might want to find another danmkau renderer that supports this, or implement one myself. Implementing this outside of the renderer is not good UX.

[Mr-Quin]

Added the options in 4f8c75d

[Mr-Quin]

Problem with danmaku engine continued in #9