Add option to disable collapsing of directory names

[juanibiapina]

This pull request adds the following option: NERDTreeCascadeSingleChildDir,
with default value of 1 (in order to keep the current behavior).

It can be disabled to prevent directory names from being collapsed into the same
line.

It doesn't affect the behavior of NERDTreeCascadeOpenSingleChildDir, because
it only changes how directories are displayed, not opened. Both options can be
used together in any combination.

After making the changes I realized "cascading" can generally be replaced with
"collapse", except when referring to "cascade open". I decided not to make this
change on this pull request.

[juanibiapina]

@scrooloose

[shannonmoeller]

@scrooloose Please merge this, or implement similar.

#76

[nmhnmh]

@scrooloose please merge this, I really need this

[hookdump]

Thank god this option exists. How is "1" the default value for this? The option does not work. I cannot open directories that have been collapsed.

[lifecrisis]

@hookdump, you don't need to open directories that have been collapsed because there is nothing else in them. This is a very useful feature in a number of programming contexts. It still has bugs, but collapsing directories by default is not one of them.

[juanibiapina]

@hookdump The default value is 1 to keep backwards compatibility.

[shannonmoeller]

@PhilRunninger Thank you for merging this! Hadn't noticed until @hookdump commented. Soooo much better.