Handle first child specially

[jakirkham]

Closes https://github.com/mbr/asciitree/issues/10

Adds first_child_head and first_child_tail to Style and implements them in a compatible way for BoxStyle. Updates LeftAligned to handle the first child with these special methods.

[jakirkham]

Still need to resolve the case of having only one child. Namely does it go through first or last or possibly both and why.

[jakirkham]

Made the only child case a special case that can also be overridden.

[jakirkham]

Also updated all styles to define these functions in a way that is compatible with their existing behavior.

[jakirkham]

While this technically works and passes the tests, I'm not sure that it is helpful for solving my problem yet. Am trying to generate some HTML and CSS to make a visualization like this one (item 4) The code being generated needs to look something like this, but was having a hard time getting that behavior out of these functions. Ended up rolling my own function, but it would be better if that wasn't necessary.

[jakirkham]

Opened issue ( https://github.com/mbr/asciitree/issues/12 ) based on the last point. Namely handling cases of having or not having children specially.