Support for generation of project tags file

Considering the number of project files spread in different locations
when developing an Arduino project, proper use of tags can be difficult;
resolving beyond local functions.

I've added automatic generation of a tags file, which includes:

* Standard ctags source in project dir (.c, .cpp, .h)
* Arduino source in project dir (.ide, .pde)
* Arduino core based on detected project core from Arduino install.
* Included Arduino libraries from user library folder.

As a Vim user I find this hugely useful and think it would be a useful
addtion for others. Target has been added as `make tags`.

1 files changed, 52 insertions, 0 deletions

diff --git a/arduino-mk-vars.md b/arduino-mk-vars.md
index 0df7a11..bef58bd 100644
--- a/arduino-mk-vars.md
+++ b/arduino-mk-vars.md
@@ -11,6 +11,7 @@ The following are the different variables that can be overwritten in the user ma
 *	[Avrdude setting variables](#avrdude-setting-variables)
 *	[Bootloader variables](#bootloader-variables)
 *	[ChipKIT variables](#chipkit-variables)
+*	[Ctags variables](#ctags-variables)
 
 ## Global variables
 
@@ -1402,6 +1403,57 @@ MPIDE_DIR = $(HOME)/mpide
 
 ----
 
+## Ctags variables
+
+### TAGS_FILE
+
+**Description:**
+
+Output file name for tags. Defaults to 'tags'.
+
+**Example:**
+
+```Makefile
+TAGS_FILE = .tags
+```
+
+**Requirement:** *Optional*
+
+----
+
+### CTAGS_OPTS
+
+**Description:**
+
+Additional options to pass to `ctags` command.
+
+**Example:**
+
+```Makefile
+# Run ctags in verbose mode
+CTAGS_OPTS = -V
+```
+
+**Requirement:** *Optional*
+
+----
+
+### CTAGS_CMD
+
+**Description:**
+
+Location of `ctags` binary. Defaults to user path.
+
+**Example:**
+
+```Makefile
+CTAGS_CMD = /usr/local/bin/
+```
+
+**Requirement:** *Optional*
+
+----
+
 ### MPIDE_PREFERENCES_PATH
 
 **Description:**