Implement loading .alda file from within the repl

[hammerandtongs]

Implement (or Document) how to load and reload a .alda file from within the repl

This would be of immediate help in lowering the startup time for many users (30 seconds for me atm).

[daveyarwood]

This is a planned feature of the Alda REPL, which we will be improving in the near future.

You will soon be able to load files by typing :load filename.alda, which will replace whatever score you may have been working on with the score loaded from the file. Then you'll be able to play the entire score by typing :play. (This isn't implemented yet, but will be soon!)

We will be tracking this feature and other Alda REPL-related features in #54.

[daveyarwood]

@hammerandtongs I ended up adding this feature tonight! I've just pushed alda v0.5.0, which adds the :load <filename> REPL command.

You can also now run :score from the REPL and see a representation, in Alda code, of the current score.