Add width option

[Caesarovich]

Hello, here is the awaited width option. ( fixes #51 ). This PR comes with a little re-arrangement of the code. I moved all the code responsible for determining how to resize/shrink/disable options into a function determineDimensions. From now on, code that handles resizing, shrinking, etc... Should be place in that function.

Features:

• The width option allows to set a fixed size for the box. This is useful in the cases where a user wants a box to be rendered regardless of current terminal dimensions.
• Bug fix: padding overflow. If there wasn't enough space to fit the text (at least one character per line) inside the box, padding would overflow on the side. (This issue has never come before because of the low probability of a padding to be larger than terminalColumns - bordersWidth - 1)

Thanks for reviewing !

[Caesarovich]

I'm too used to use it my bad.

[sindresorhus]

Suggested change

	Set a fix width for the box. (This disables terminal overflow safeties !)
	Set a fixed width for the box.
	*Note:* This disables terminal overflow handling and may cause the box to look broken if the user's terminal is not wide enough.

[sindresorhus]

Suggested change

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[Caesarovich]

Very well, you're much better than me at making meaningful phrases ^^'