Implement verbose messages

Verbose messages can be useful for tracking luatodo's operations.
Four types of message have been implemented, each with a leading symbol that
makes luatodo output easy to parse with external programs:

- "File found" : [*]
    luatodo found a file that matches the given extensions. These messages can
    be filtered using sed:
        sed -n '/^\[\*/p'

- "Note added" : [+]
    luatodo found a complete note (i.e. tag, timestamp and description). These
    messages (two lines each) can be filtered using sed:
        sed -n '/^\[+/ {N;p}'

- "Note added - missing timestamp" : [=]
    luatodo found a note without a timestamp (i.e. tag and desription). These
    messages (two lines each) can be filtered using sed:
        sed -n '/^\[=/ {N;p}'

- "Note ignored - missing description" : [-]
    luatudo found a valid tag but didn't find any description for it. These
    messages (two lines each) can be filtered using sed:
        sed -n '/^\[-/ {N;p}'