Support relative paths with and without specified filenames

[BrianHenryIE]

Specifying the output directory was not working as expected, as discussed in #34. e.g. "wp dist-archive . ./scratch/build.zip --create-target-dir succeeds but the file is not placed in the subdirectory of ., rather it's at ../scratch/build.zip"

Without a specified path or filename, the default output folder is the plugin's parent directory with the generated (versioned) filename. Without a specified path, the default output folder is the plugin's parent directory with the specified filename. With a specified path and filename, the archive is saved to that path and filename. With a specified path and no filename, the archive is saved to that path with the generated (versioned) filename.

Tests added for:

• wp dist-archive . hello-world.zip – output filename, no path
• wp dist-archive . ./hello-world.zip – output filename, with . for path
• wp dist-archive wp-content/plugins/hello-world wp-content – relative output path, no filename
• wp dist-archive wp-content/plugins/hello-world ./subdir/hello-world.zip – relative output path, with filename
• wp dist-archive . {RUN_DIR}/hello-world.zip – using . as input and absolute output path, with filename
• wp dist-archive wp-content/plugins/hello-world . – using . as output path, no filename

No documentation changed.

Fixes #34

[BrianHenryIE]

Related to issue #48, PR #49.