sourcelookup: allow more than one input sheet (cf. #70)

[johenglisch]

grambank sourcelookup now takes more than one data sheet as its argument, so one can process multiple sheets, while only having to wait for the bibliography to load once.

@HedvigS About your idea of reading the file paths from a txt file: You can still do that by running grambank through xargs(1).

$ xargs grambank sourcelookup path-to-glottolog-repo < list-of-files.txt

Only downside is that you have to escape all spaces within the file names, e.g.:

/home/User\ Name/Desktop/My\ Data\ Sheets/XYZ_abcd1234.tsv
/home/User\ Name/Desktop/My\ Data\ Sheets/ABC_xyzw1234.tsv

[HedvigS]

grambank sourcelookup now takes more than one data sheet as its argument, so one can process multiple sheets, while only having to wait for the bibliography to load once.

That's so great, thank you Johannes!