Support custom field separator

[adyanth]

Fixes #30 with a new environment variable SPLITWISE_FIELD_SEPARATOR.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
main.py	240	75	68%	103, 107, 114, 118, 122, 137, 147, 159, 185–188, 242–245, 247, 249–252, 254–256, 258–259, 262–265, 267–268, 270, 273, 275–282, 297–299, 304–307, 309–310, 333–334, 342, 344–345, 382, 419–420, 422, 426–429, 471, 475, 478, 480, 482–485, 487–488, 490
TOTAL	240	75	68%

Tests	Skipped	Failures	Errors	Time
21	0 💤	0 ❌	0 🔥	2.792s ⏱️