Add support for conditional formatting

[domsson]

Currently, the biggest disadvantage of succade + lemonbar over other solutions, like polybar, is that it isn't possible to have conditional formatting. For example, you might want to have the CPU temperature string be displayed in blue for low temperatures, yellow for mid-range temperatures and red for high temperatures. We can't do this at the moment.

Here is an idea of how this could be implemented:

• The configuration files for blocks (or a global config, doesn't matter) could make use of INI sections to list alternative settings. Each section would denote a mode of the block.
• Blocks could print mode change requests to stderr, and succade would then make use of the appropriate config section of that block.

Example block config:

[temperature:default]
bin = "temps -m"
foreground = "#ffff00"

[temperature:low]
foreground = "#0000ff"

[temperature:high]
foreground = "#ff0000"

Example stderr output of temps -m when a certain temperature change is detected:

@mode:high