Fix variable names with digits, ERL, update docs

- Tokenizer now copies entire identifier (letters+digits) when no
  keyword matches, fixing variables like K2, V1, AB3%
- ERL returns actual error line number instead of 0
- Update README and Sphinx docs for v0.5.0

tests/programs/error_handler.bas

@@ -7,5 +7,5 @@
 70 X = 1 / 0
 80 PRINT "After second resume"
 90 PRINT "Error handler OK" : END
-100 PRINT "Caught error"; ERR
+100 PRINT "Caught error"; ERR; "at line"; ERL
 110 RESUME NEXT