Add support of reading parameters for external editor/viewer from main config.

Add support of reading parameters for external editor/viewer from main config.

for example, the config file may contain strings:

[External editor or viewer parameters]
    vim = %filename +%lineno
    mcedit = %filename:%lineno

And these external editors will open a file at the previously saved position.

Signed-off-by: Slava Zanko <slavazanko@gmail.com>