Add gitEdit for invoking the default editor

Ref #1
Implement gitChangelog

Ref #1
Implement changelog generation

Close #1
Generate changelog from previous <release> version

For example, when releasing from v0.2.1 to v0.3.0, the changelog will be
generated from v0.2.0 to v0.3.0.
Fix some prerelease-related behavior
Add manpage and makefile target for it
Ignore compressed manual files
Change commandline arguments and add ref support
Update manpage for new options and arguments
"make clean" now removes man pages