add-on readers in 1.31

[simonmichael]

An old goal, and I think 1.31 is the time to add it: hledger-read-EXT scripts in PATH allowing hledger to read EXT files. Like add-on commands, but for adding support for new data formats.

First draft spec:

• an executable in PATH named hledger-read-EXT
• which should take one argument, a file path with the .EXT extension
• and output a conversion as hledger-compatible journal format on stdout
• will be listed in hledger's commands list or help as an add-on reader
• will allow hledger -f ANYFILE.EXT CMD to work
• hledger will have to parse the journal format in the usual way, which is not optimally efficient but simple and robust for now

If anyone wants to work on it before me, please do.