Misc. enhancements

[kashav]

Various enhancements and bugs that I've come across, feel free to comment below (or create a new issue) for suggestions or bugs.

• Bug: Exclude skips files with similar names (e.g. excluding .git results in .gitignore not being listed).
• Add unit tests (test files are empty right now).
• Add support for regex in string comparisons (e.g. ... ENDSWITH jsx?).
• Handle errors more gracefully (instead of just panicking everything).
• Add support for OR / AND / () (for precedence) in condition statements (lexing is already done for these, just need to add the parsers).
• Add support for times/dates (to query file creation/modification time).
• Introduce new attributes to select from (creation/modification time, file mode, basically whatever else os.FileInfo supports).
• Bug: Space-separated queries. Currently something like "... WHERE time > May 1 ..." is broken since we're splitting conditionals by space. Fix by allowing single quotes and backticks in query strings, so something like "... WHERE time > 'May 1' ..." works and evaluates the conditional to have value of "May 1".
• Add NOT operator for negating conditionals.
• Add support for querying and selecting using other size units (only supports bytes right now, add functionality for KB, MB, and GB as well).
• Bug: Selecting from a directory and it's subdirectory results in duplicates and malformed output.

Moved from the README to this issue as of b78a987.

[kashav]

Closing in favour of individual issues. See #5.