set_relative_date was failing if the resulting date was in the previous
day.

Following PR #19

And updated doxygen todo list

And bugfixes in _ttail_set_date_relative() bad format detection

Adds the option -P --permissive.

Starts implementation for logfiles.

Now -P flag make ttail handle invalid files. When invalid file given the
minmax function set the file pointer and the minmax array to NULL.

May introduce bugs if the binary search function does not handles "holes"
in fp array or in minmax array...

Telling getopt to produce an error message by setting opterr to 1

Not done for stdin

Making this function usable by logfiles and stdin format autodetection

Date formats

        Two date formats are allowed for dates arguments (-d , -m) :
        - the same than in the logfile
        - relative from now prefixing the argument with '#-' and given a unit
                - y : year
                - M : Month
                - d : day
                - h : hour
                - m : min
                - s : sec

Now ttail return an error if the whole date given in argument is not
matched by the format

Now an error is triggered if date-min > date-max
Bugfix on date-max searching (on multiple loglines with a date == date-max)

add a -h --help option, deleted -l --logfile options
+ spliting the ttail_search_closest() & ttail_search_print_res() functions

To handle case where none of the file given as argument is readable

Now we have a running minimal ttail :
src/ttail -d "Mar 6 00:29" -f '%B%n%d %H:%M' -l tests/samples/1.log -l tests/samples/1.1.log

ttail_set_flag_re_ex() and ttail_set_flag_re_ci()

For extended & case insensitive regex

Now we can specify a constant length to cut