Test for lexer/parser

[alienscience]

The lexer and parser are likely to have more bugs in them. It would be good to have a test that throws valid and invalid commands at the lexer/parser and checks the results.

[EtienneBruines]

There have been several commits regarding lexer_test.go. What are the criteria for this being finished?

Perhaps referencing in commit messages, like Added tests for lexer, see #14 would make this easier to find.

[alienscience]

There are few tests at the moment because the Lexer and Parser are unfinished. There are some IMAP commands that will require big changes to both the lexer and parser. This issue will likely be open for a long time.