Merge pull request #7 from nilbus/glob

Avoid an error on load when the composure dir does not yet exist
Only attempt using readline for input in bash shells

ksh does not support readline with read.
zsh supports readline with varedit, but its input cannot be fed with a
pipe, which causes the test suite to hang, waiting on the tty's stdin.
Relax test for _add_composure_file

this test specified that _add_composure_file should accept a comment on
STDIN. In practice, this feature was unused and blocks a future
enhancement.

Leaving the second test commit as an integration check to ensure that
back-to-back commits function as expected.
Refactor prompt_cmd() into prompt() to avoid eval

The prompt must be displayed on stderr, or else it will become part of the result,
since stdout is used to pass the result back to the caller of _prompt().
Merge pull request #8 from nilbus/readline

Use readline for reading git commit messages