Replace hardcoded executables with variable
Fix whitespace issues
Add a warning when HEX_MAXIMUM_SIZE is not specified
Add the ability to configure avrdude options

A new environmental variable AVRDUDE_OPTS is added, which can be used to
configure the options passed to avrdude.

Fix #53
Handle cases where certain fuse bits are not present

Introduce two new variables AVRDUDE_ISP_FUSES_PRE and
AVRDUDE_ISP_FUSES_POST which are set based on whether the fuse bits are
present or not.

avrdude is invoked to set fuse bits only if these new variables are not
empty.

Fix #61
Add support for compiling plain AVR C files

To compile plain AVR C files, the variable NO_CORE should be initialized
to a non-empty value.

When NO_CORE is set, the following changes happen
- boards.txt file is not parsed. The user should explicitly set MCU,
  F_CPU, fuse bits etc.
- MONITOR_BAUDRATE is not calculated

Fix #63
Add an example to show how to compile AVR C files
Release v0.11.0