v0.3.0: battery background and interval ms

The battery block can change its background color if the level becomes critical.
The global configuration option interval is now in milliseconds instead of seconds.
Release builds are lighter thanks to lto optimization.

See docs/features.md for details.

v0.2.0: cpu_freq, cpu_temp and memory

This new release brings the following new modules:

• "cpu_freq" displays the frequency of the CPU
• "cpu_temp" displays the temperature of the CPU
• "memory" displays the memory used by the system

It also adds the "interval" configuration option to decide how often the bar updates.

A few minor internal API changes invisible to the end user, like the improvement of the C headers ports. This way the program is ready if one day these headers are merged in the libc crate.

v0.1.0: First version

This first version defines a clear architecture to be able to add blocks and support operating systems easily.

Two blocks have been defined so far: datetime (multi-OS) and battery (OpenBSD-only).
Since sysctl has been implemented, adding new blocks for OpenBSD should be easy.