Allow tempo to be set by length of beat in ms

[Damien Clarke]

Complements (bpm:...). Considering (bms:...) for "beat milliseconds", so the first letter alphabetically groups it alongside bpm, and milliseconds used rather than seconds because

• time spans will tend to be short
• seconds aren't hard to represent as milliseconds
• it avoids a limit of decimal point that may give a false sense of accuracy, and
• I'd like to avoid having alternate ways of writing the same node to reduce the width of the lexicon and amount of docs

[Damien Clarke]

Released in 1.4.0