Introduce preferredBufferSize parameter to allow custom buffer size when streaming subprocess output

[iCharlesHu]

Currently, Subprocess always uses the same buffer size (system page size). This is an issue for executables with small but frequent outputs because Subprocess will always wait until it gathers buffer-size-bytes before it returns the read byte., causing streaming delays. Introduce the preferredBufferSize parameter to allow buffer size customization. Developers could choose a smaller buffer size if they anticipate that the output from the executable will be small.

resolves: #39

[jakepetroules]

LGTM other than the docs typo (or parameter name type).