Add HISTORY_LINES for user overrides
Add HISTORY_LINES for user overrides

Use arithemetic expansion [1] to increment that value automatically,
and add more detail on why we need the increment.

[1]: http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_04

r-gaia-cs/swc-shell-split-window,rgaiacs/swc-shell-split-window