Generalize and move parsing of reflog info into main_read
Simplify checking of --stdin argument
Refactor the pager mode handling

Remove the global opt_stdin variable and the VIEW_STDIN flag and move
checking of pager mode options and opening logic into open_pager_mode().
Two new flags (OPEN_PAGER_MODE and OPEN_FORWARD_STDIN) is added to deal
with how the IO for the view needs to be initialized.

Also fixes a bug, where stdin was not closed when pager mode is not
supported. This means that the process writing to stdin would hang until
Tig closes.
Load main view from any --pretty=raw output given on stdin

This allows you to run:

    git log --pretty=raw | tig --pretty=raw
    git reflog --pretty=raw | tig --pretty=raw
    git stash list --pretty=raw | tig --pretty=raw
    git rev-list jonas/code-split --pretty=raw | tig --pretty=raw
    git whatchanged --pretty=raw | tig --pretty=raw

Idea and commit message by Vivien Didelot.
tig(1): document the Git commands supported by the pager mode

Also add some general improvements to the manpage description, add the
stash subcommand and more.

Fixes #1