Support substring and replacement #8

[pombredanne]

This PR adds minimal support for some Bash idioms for #8

• simple replacement as in ${foo/bar/baz}
• substrings as in ${foo:4:2}

In addition:

• add support for plain $parameter substitution including some simple nesting.
• apply some minor code cleanups

Signed-off-by: Philippe Ombredanne pombredanne@nexb.com

[pombredanne]

Note that I have tried to keep each commit mostly self-standing if you want to cherry pick some

[pombredanne]

Thank you for the swift merge.