Start insert mode if LogpadInsert is set to 1 but do not make it default

Making insert mode default is not required. All user wants is to be in
insert mode when LogpadInit() is called. But that does not mean insert
mode should be made default.
Fix a bug in timestamp format

strftime('%c') returns current date. It could have a single digit date
or double digit date. When date is single digit a space is added before
the date. So you get two spaces, instead of one, after month.

Logpad relies on the timestamp format to decide whether to add timestamp
to file or not. If timestamp is not constant result will be variable.

This commit removes this ambiguity and adds a constant timestamp that
handles single and double digit dates without any hitch.
Add option to insert time elapsed since last timestamp under the new timestamp
Change README to markdown
Update documentation