Handle equals and allow typing texts in cells

[ryonakano]

Fixes #39
Fixes #22

[elegaanz]

Looking at the code, it would still accept formulas that don't start with a =. Maybe it should behave like in other spreadsheet apps: if it starts with an equal sign, what follows is a formula, otherwise it is interpreted as verbatim text or number. What do you think?

[ryonakano]

You're right, that would be more reasonable. I'll work on that too.

[ryonakano]

Now it's possible to type texts like Hello, how are you? or Hi!, but when you type 12 days (contains both numbers and texts), the cell displays an Error. Also, when = is not used before a function like sum(12,23) or 12+9, it should display as a plain text like Excel as you mentioned.

[elegaanz]

It looks like it interprets more often than what Excel/LibreOffice does (i.e 2+2 will display 4, not literally 2+2, even if there is no =), but it just feels a bit more smart IMO, even if it could be confusing because some formula will require a = while others won't.

[elegaanz]

👌