fix parsing of priorities tsv file to allow spaces in sequence IDs
fix parsing of priorities tsv file to allow spaces in sequence IDs

When parsing priorities.tsv files, an error could be encountered if a sequence name contain spaces, as `.split()` breaks not only on the tabs delimiting the file, but any whitespace character. This repalces `.split()` with `.split('\t')` to address this and preserve sequence names containg spaces.

blab/nextstrain-augur,nextstrain/augur,nextstrain/augur,nextstrain/augur