Fixes unnamed append points.

In `ConfigFile.compile()`, the variable `content` is used but not
initialized, which results in unnamed append points being deleted and
`insert_lines` being appended at the end instead. Due to the
undifferentiated `try/except` block around this, the error is silently
ignored.

This fix makes the `try/except` block only handle `ValueError`, which
would occur when there's no explicit append point.
Merge pull request #11 from runiq/fix-unnamed-append-points

Fix unnamed append points
Make SOURCE_DIR configurable. Fixes #6.

For now, this is a command-line switch. Allowing the use of an
environment variable (`DOTS_SOURCE` probably, which is already used when
calling install scripts) would probably be better, but one thing at a
time...
Merge branch 'configurable-source-dir' of https://github.com/runiq/dots into configurable-source-dir