Allow Strings for TableStyle delimiters.

[Xitian9]

Fixes #17.

This allows a greater range of delimiters allowed in tables, including multi-character delimiters like ||, or null delimiters "". This furthermore allows us to create tables with no outer borders.

Here are two examples of new table styles created with this.

Using asciiDoubleS:

├────────────────────────────┼───────────────────────────────────────────────────────────────────┤
│                            │        ++-------------------++-------------------++-----++        │
│  len spec: expand          │        ||     Some text     ||   Some numbers    ||  X  ||        │
│  position: left            │        ++-------------------++-------------------++-----++        │
│ alignment: no align        │        || This is long text || 4.20000000        || foo ||        │
│                            │        || Short             || 200300400500600.2 || bar ||        │
│                            │        ++-------------------++-------------------++-----++        │
├────────────────────────────┼───────────────────────────────────────────────────────────────────┤

Using withoutBorders unicodeS:

├────────────────────────────┼───────────────────────────────────────────────────────────────────┤
│  len spec: expand          │                Some text     │   Some numbers    │  X             │
│  position: left            │           ═══════════════════╪═══════════════════╪═════           │
│ alignment: no align        │            This is long text │ 4.20000000        │ foo            │
│                            │            Short             │ 200300400500600.2 │ bar            │
├────────────────────────────┼───────────────────────────────────────────────────────────────────┤

[Xitian9]

I think I've addressed all the comments. Let me know if you'd like other changes.

[muesli4]

Otherwise it seems fine to me. After those small changes are done I will merge it.

And I wanted to take the chance to say thank you again for your work and your patience. I know how much it sucks having to wait for a review on open source projects. I promised myself to never do it this way but I guess I understand the reasons why this happens now. ;)

[Xitian9]

Updated. Let me know if that's satisfactory.

And don't worry about the wait; thank you for maintaining this wonderful library!