Formatter can be configured via options

[borgar]

Adds options and behaviour for:

• Overflow error string (switching to ###### from #VALUE!)
• Have formatters throw when there are date overflow errors (SSF does this)
• Have formatters switch to General when dates overflow (Sheets does this)
• Allow a wider range of dates (choice between Excel's 1900–9999 or Sheet's 0–99999).
• Emulation of the "1900 leap year bug".
• Have formatters emit regular spaces rather than non-breaking ones.
• Disable parse-time throws (was an argument).
• Error string emitted when formatter has been handed an invalid date and throws disabled (######).
• Locale (was an argument).

Still to-do:

• Should the 1900 leap year bug be on by default (useful for spreadsheet software but not for any other potential users).
• Should range be limited to the narrower one by default (useful for some spreadsheet software but not for any other potential users).
• Would like to optimize the NBSP conversion by flagging parts that actually need it.

Closes #3 Closes #4

[borgar]

Should the 1900 leap year bug be on by default (useful for spreadsheet software but not for any other potential users).

Calling it as yes.

Should range be limited to the narrower one by default (useful for some spreadsheet software but not for any other potential users).

Calling it as yes. Narrow is only really useful for Excel emulation.

Would like to optimize the NBSP conversion by flagging parts that actually need it.

Leaving this for later.